Use a screw driver if you have one to depress the little button. Choose one just small enough to fit the slot. This will limit how far downward the release can be pushed. Using something with a point can knock the keyless works out of alignment requiring the face and hands to be removed to rectify.
(I have had to do this more than once)
When you reinsert the stem, dont force it. I like to depress the release button and press the stem back in while gently twisting it...it slips right in that way.
While you are at it you can go to Lowes and get some Gunk silicone grease from the plumbing section. Comes in a little round container...use that to lube the gaskets while you have the stem out, there are three. One inside the stem tube, one inside the crown, and one on the outside of the stem tube. Also grease the gasket on the case back and that should make your watch water tight. I tested my DSSD to 16 ATM (160 Meters or approx 530 ft) and I beleive it could do much better than that. So long as your crystal is properly pressed with no issues with the crystal gasket, it should be just as water tight.