If your not familiar with removing the stem on an A7750 movement just leave it like it is for the moment! Ask your dealer whether he can source you a new stem together with the crown. I am sure he will As soon you have both send it to a modder or go to a local (repfriendly of course) watch smith to replace it. Basically, he will take out the broken stem and put in the new one. Very easy to do if you know how but removing/inserting the stem without knowing how to do it correctly can mess up your movement and then things are getting complicated. Nevertheless, if you feel comftable diy but I would recommend to do the 1st option. Good luck!