First there is absolutely no assurance that a second router will provide a better or more powerful wireless signal than the first. It most likely will not as these units, regardless of maker, use the same broadcast power. Now if you wish to relocate the second device closer to upstairs and connect via a cat5 cable, then your success prospects rise markedly. However, placing the Belkin close to the original router will do little to nothing to assist you.
Having said the above, if you wish to install the Belkin anyway, get the manual out and read it as it will describe exactly how to do the needed steps. There is not enough room in this forum to reprint the manual.
Here is what I would consider doing:
1. Disable wireless on the Sagem router as it apparently is not helpful. Now if other users are successful with wireless link to Sagem, they are probably positioned closer to it. In this case, leave the wireless link live and determine the specific channel it uses and record it in your documentation (you must document in writing what you are doing or you will end up with chaos; I know people are reluctant to document but this reluctance promptly becomes major trouble so document; I usually have either Word or Notepad open and document as I proceed as this is an easy way to do the job). Record the default gateway IP address, DHCP range, subnet mask of the LAN side of this device. To do these you must log onto Sagem using the default gateway IP address as the address in your web browser and you need the user name and password. You must use a pc that is linked via a wired CAT5 cable to the LAN side of the Sagem to do this, never a wireless link. To get the default gateway, open command prompt (start, run - type cmd and hit enter), and at the command prompt > prompt type in "ipconfig /all" without quotes. Record all the information it provides and note the IP address of default gateway. Next, see if there is any static IP address in use that is next to the Sagem's address. Let's say Sagem's address is 192.168.1.1, try to ping 192.168.1.2 provided it is not in the DHCP pool, and see if you get a successful reply - if you do get a successful reply, then go up one more, 192.168.1.3 until you do not succeed - and record this. Alternatively if Sagem's address is 192.168.1.254, ping 1 below, or 192.168.1.253 and work downward until you do not have a successful ping response. Again record this.
2. Now go to the Belkin and connect a CAT5 cable to its LAN port and to your pc. Do not connect Belkin WAN port to the network yet. Boot up the Belkin and reboot your pc. Determine your default gateway IP address as well as the other address info and document it. just like you did above. If the default gateway of the Belkin is the same as for Sagem (e.g. if both are 192.168.1.1, mask 255.255.255.0 or both are 192.168.0.1, mask 255.255.255.0 they have the same IP address) or if they are on different subnets (same subnet mask but, for example one is 192.168.1.1 and the other is 192.168.0.1) we have some changes to make on Belkin as follows:
2.a. - If they are on the same subnet (e.g. both are 192.168.1.1 or both are 192.168.0.1, or in fact if the first 3 sets of numbers are the same with subnet mask 255.255.255.0) they have the same subnet which is what you want. However the last set of numbers must not be the same. If both are on same subnet with same last set of numbers, change Belkin to a number that is not in the Sagem DHCP range and also not used as a static LAN IP by another LAN device (e.g. let's say Sagem uses 192.168.1.0 subnet with the 255.255.255.0 subnet mask, Sagem has IP 192.168.1.1, Belkin uses IP 192.168.1.1 also, and Sagem DHCP range is 192.168.1.50-192.168.1.100, I would want to assign Belkin the IP of 192.168.1.2, mask 255.255.255.0 provided it is not in use as reveled in ping test in step 1 - as shown in Step 1 we want an IP assigned to Belkin that is in the same subnet and also not used by DHCP pool and not assigned to another device). See 2c before you change Belkin IP.
2.b. If Belkin has a different subnet, it must be converted to the same subnet as Sagem. Let's say Sagem is 192.168.1.1 and Belkin is 192.168.0.1 - then Belkin must be changed to 192.168.1.x and x cannot be 1, cannot be in DHCP pool of Sagem, and must not be in use. If you followed my directions thus far and wrote them down you know what to use.
2.c. Now on Belkin change the IP address to be on the same subnet and with a different IP address in the last number set. Note the assigned IP address. You will lose connection to Belkin and have to relog on using the new IP address you just gave it. Once back on Belkin after IP address change, disable the DHCP capability of the Belkin device. Save the Belkin router settings or they will be lost at next reboot. If you can download the Belkin configuration to your pc, do so and note exactly where you saved it.
3. Now both units are on same subnet, h