Most simple way:
1. Download firmware archive from here:
http://yadi.sk/d/svKHEN8iDdKGW2. Unpack it using any archiver like winzip or winrar (google it if you don't have it preinstalled on your PC). You will have one big "update.img" file.
3. Make sure your S7800b is fully charged and microSD card with at least 1 GB free memory space installed in its microSD slot, then connect it to PC using USB-cable.
4. When you plug USB-cable both to PC and S7800b, on S7800b will pop up the connection screen with Android mascot. Tap on Connect button on that screen and mascot will change its color indicating successful connection.
5. Now go to your PC "My computer" and select newly appeared disk - a microSD inside your S7800b. Make sure it's microSD and not internal storage of S7800b (check files, on new or freshly formatted microSD there is no single file). Then put update.img right on microSD's root (in other words, just drag it directly to the SD card in "My computer").
6. On S7800b screen tap again on same button from step 4. When Android mascot changed its color you may unplug USB-cable from your S7800b. Plug in charger.
7. Now system will detect firmware update and will ask you if you want to update. Tap on Yes and just watch the process - it will reboot with animation of Android robot with cog-wheels in its belly and update indicator below. It will take few minutes to finish the process and after it's done, S7800b will reboot as usual and then you may enjoy your new firmware. System will ask you if you would like to delete update file from SD, agree.