Depends on which phone you have - Samsung and (I think) HTC have software suites that come with the phone that will allow you to update via a USB connection to a PC.
Other phones and tablets like the Nexus line don't have this: for bigger updates, they sometimes require WiFi though often, at a later date the update will be made available OTA (over the air) via your mobile network connection (note that some updates can be quite big so you need to make sure the data allowance on your contract covers this).
If you don't have WiFi at home, you could look for somewhere that provides free WiFi like some coffee shops for instance.
