If you want to change the computer that it's synced to then see if this user-tip helps : https://discussions.apple.com/docs/DOC-3141
But if you just want to add music from that new computer but not change the computer that it's synced to then see this page : http://support.apple.com/kb/HT1202
You can have your account authorised on up to 5 computers, that allows you to buy and use your content on those computer (e.g. play music, watch films). But some devices are designed to only be synced to one computer at a time, so syncing to a new computer would replace the current contents of the device with what is on the new computer.