What watch do you have? 364, 285, 194? There are more 2500m models.
I suppose you have strap change system like on PAM 243, with little button on one of the lugs on each side of the watch (on back side of the lugs of course). You have to push that button with plastic strap change tool or with toothpick and while holding it pushed, in the same time push the pin to the outside. Not sure if you understand what I mean since english is not my first language but to make it easier, just check here: https://www.youtube.com/watch?v=kkyWsaNh99s