Can anyone tell me how to change the serial on a dns zone file ?

I can manually edit the file, however whenever i make a change in the gui, it changes the serial number back to previous one (just incremented from there, instead of incrementing the new serial)

Any ideas would be greatly appreciated.

 

That's not possible ... Plesk controls the serial numbers (which are timestamp-based). You cannot change them yourself without Plesk resetting them.