So, I have a 245M. I do trail running, wiped out, smashed it up. Ordered a crystal, coming from China in *months*. So, I saw a 245M on ebay cheap as "defective, won't power on". Thought I'd take a gamble on maybe the magic-button-releasing-hard-reset was all it needed. But, the reason it wouldn't power on, turns out to be the power button sticks hard in the "in" position. Took off its glass, put it on my smashed one, my original 245M is now fine with the display from the ebay one, but then I set out to fix the button on the other one.
SO... if you want to FIX the button, be good with tools and magnifiers and tiny tweezers... here is what I found.
The button has a shaft with an o-ring and a flat spring-clip retaining it and a spring. Strangely, the issue is NOT with the o-ring or shaft in the piston or the spring - it is that the top wide part of the button is cast too wide and abrades the plastic watch housing, at the full-button-width part, not the shaft with the o-ring. This can be fixed ...
Step 1: Remove watch glass. Heat, pry (behind bezel, not at glass edge). Careful of ribbon connector.
Step 2: Remove system board, unscrew first with torx, remove ribbon connector for wrist sensor.
Step 3: Remove spring clip on inside of sticky button. This is tiny and will want to go flying, careful.
Step 4: Remove button by pushing from inside, don't lose the spring either.
Step 5: Put button shaft including o-ring in tiny dremel chuck and spin on sandpaper or file.
Remember, the goal is not to affect the shaft or o-ring, just the WIDE part of the button.
Step 6: Test button clearance for smooth sailing, repeast step 5 if necessary.
Step 7: Reverse 1-5 and use E6000 adhesive to seal glass/bezel watertight (remove old adhesive).
OK, so, this is maybe a bit hard-core - but - I had the extra watch with stuck buttons open with the glass off, having used the screen to replace the one on the one I smashed on the trail.
Recommended tools: Magnifier headthing, bright light, tweezers, tiny torx and metal spudgers, E6000 cement, silicone lubricant, small drill chuck with drill or screwdriver or dremel, file or sandpaper.