You will want to manually update to 3.60 then.
The easiest way to do this on a PSTV is to place the 3.60 update file onto USB storage media (place it at PSVITA/UPDATE/PSVUPDAT.PUP) and after that, shut down your PSTV, then boot into safe mode by holding down the power button for at least 10 seconds (or until safe mode appears on-screen).
You should see an option to update the system software. Select that option, and then select "update using storage media". Insert your USB storage into the USB slot in the PSTV, and the 3.60 firmware should hopefully be detected and copied over to the internal storage ready to be used.