How to open the back of a wrist watch to change the battery. i have two disney watches both have red m on the litle dial you use to set the time the model numbers on back of the watches are: mu1240 mu1108 both are water resistant disney by sll marketing international

Identify first if it is a "screw on back" or a "pop off" back. Screw on backs have multiple indentions on the caseback where a special tool fits in the grooves to twist it on and off. If it is a screw type back then you will either need to buy a pricey caseback wrench or take it to a jewelry store that changes watch batteries.
A pop off back will have a smooth caseback with no indentions on it.
Most of these are "pop off" backs. to pop the back off get a very sharp blade and apply pressure at about a 30degree angle between the case and the caseback on the underside of your watch. From there you should be able to pry it and it should pop right off. Another way is to put a razorblade between the case and caseback, tap the blade with a small hammer (lightly!) and then pry the back off.