If you want to download the firmware and install it over an USB connection, see Patrick Gaskin's
Kobo Firmware Downloads page. Please note that any of the 3 flavours of the Kobo Touch can be updated to the current 4.38.21908 firmware but you need to correct version for your Touch. The Touch N905 and Touch N905B use a different updater from the Touch N905C. The N905 is sometimes referred to as a N905A.
You should be able to find the model in Settings => About.