Is this a DIY project?

What do you see as a problem?

It looks like the trex is attached with screws. After removing, the boards should slide out. You'd leave the flashing and slide new boards under it.

Agree, you can see the end fasteners clear of the flashing. If for some reason the flashing causes a problem we can't see here, you can use hidden fasteners (attaches to the side of boards) at the end, or add another sleeper (assuming with balcony) as required to clear the flashing.

That looks like a rip cut to fill the gap between siding and decking. Take that piece out and remove the decking. Most likely, there is no flashing there since the joists run parallel to that wall (if there is flashing, leave it in place). If the joist is close enough, add vinyl Z-Flashing and install new decking.

I’m sure you have a good reason to replace, but if cosmetic and no design change a good cleaning would help.