Does the Lenovo ThinkCentre M83 Tower have an upgradable power supply (psu)?

Yes it has.

Looking at the two images in the source, it looks like the M83 is using a standard form factor PSU which should be user upgradable. However, before you make any changes, please note that if the computer is still under warranty, you may void the warranty by replacing the PSU. You should check with Lenovo first before making the change just to be sure that it won't void your warranty.