
appears to be a Pulsar P3 LED.
you'll find information easily at www.ledwatches.net - plenty of pictures there.
some oil or wd40 on the threads of the caseback ring for a few hours might help in unscrewing it. otherwise a jeweler should help however the proper battery size is no longer made. 357 batteries will fit with some spacers cut from a rubber pipe or an o-ring. of course if your watch was left with batteries in you might end up with a dead watch corroded by battery acid.
good luck.