During the peak of PSP ISO modding, "upd" typically referred to or cheat databases . You might find a file named Castle_Crashers_PSP_ISO_UPD.cso that was simply a pre-patched version of a different beat ‘em up—like Dungeons & Dragons: Tower of Doom (a Capcom arcade port)—with Castle Crashers textures hacked in.
However, the game is widely available on other platforms, and if you are looking for a portable experience, here is a summary based on legitimate reviews and performance: castle crashers psp iso upd
Here is why a native port never happened: During the peak of PSP ISO modding, "upd"