Is that bubbling on the dial? Question for more experienced folks: Would that be indicative of water damage/leaking oil from the movement?
If you plan on keeping it, I would find a TRUSTED watchmaker to open the watch and then service it. This will cost you some money (probably $300 and up), but is imperative to preserving the watch. We cannot be sure of the watch until we see the movement.
Water damage. It is likely that the seals are no good, the watch was worn for years in a humid environment and the bubbling was caused by condensation from temperature changes inside the watch. There is also a good chance that the movement has been damaged by these water droplets as well. gatorcpa
Go to the dollar store if you don’t have kids and buy a rubber ball. You can press it firm against the back of the watch and rotate counterclockwise. This should open it. Bottom line though is the dial needs to be replaced and if not available refurbished, the movement will no doubt need a full service. Hopefully the case isn’t damaged due to corrosion. If it’s sentimental it’s worth the investment. So sorry to see this heirloom is in need of such service.