Hello, welcome.

There are two types of updates: firmware update (big thing, you normally use computer for this) and regular software updates, usually done OtA.
It is possible to update software using computer, just download the right package and put it on your n900. But this may be troublesome and is not recommended, as it may cause compatibility problems with different packages.

You can always use WiFi to connect to the Internet from your n900 and download necessary updates.