You can update a first gen iPad to iOS 5.1.1 via your computer's iTunes
Connect the iPad to your computer's iTunes and copy any purchases off the iPad to your computer via File > Devices > Transfer Purchases. You may also want to copy photos and any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPad via File > Devices > Back Up.
Then start the update by selecting the iPad on the left-hand sidebar, and on the Summary tab on the right-hand side clicking the Check For Updates button
Updating to iOS 5+ : http://support.apple.com/kb/HT4972