[TheEplumber]

FYI- That is CPVC pipe. It is rated for this application- PVC is not.
Cut it a couple inches below the adapter. Unscrew the adapter from the valve and save.
When reassembling, use a couple wraps of teflon tape on the adapter. Purchase a 3/4" CPVC coupling and a small can of Flow Guard glue (made for CPVC) Following the instructions on the glue can, glue on the coupling at your original cut point and you're done.

Have you verified that the valve has simply failed? Or could the leak simply be a sign of high pressure or temperature within the tank?

 

[TheEplumber]

Wow- we're all over this one:vs_karate:

 

[TheEplumber]

Before you do all that on that expansion tank you will find a cap and under it a snifter valve. Hit the snifter to see if you get water out. If you do replace the expansion tank and the relief valve.

+1. IMO, the exp tank will fail before the T&P will