Flash the bootloader for an uno on it!
Normally the nano has 30720 bytes of flash available; the uno has 32256 bytes of flash. The uno exists in two versions: either with DIP or with soldered PLCC as the nano has.
For a howto see
Edit: I Forgot to mention - From now on in the IDE you have to select the uno for programming the (ex)-nano, and you should tag it as "uno" too.