Changing the battery in any watch should be done properly by a jeweler or watch repairer with the ability to pressure test for water resistance.
Not having the battery properly changed (as specified above) and doing it yourself (Removing the case back and not resealing the watch properly) will void the lifetime warranty on your Jack Mason timepiece. Please keep receipts of any outsourced watch service, in case you do have an issue with your timepiece after the fact.
The battery life of Jack Mason watches ranges from 3 to 5 years depending on the movement type. Frequent use of the chronograph functions may shorten the life of the battery.
Refer to the size chart below to find out what size battery fits your Jack Mason watch. You can find the style number on the case back. ex. JM-N102-XXX