Asking your question in full sentences and providing details on the year and model of your truck will probably get you better answers. Also, we need to know if you are just trying to replace the glass in the mirror or the entire assembly. (The entire assembly is actually easier in my opinion).
In broad terms, for the entire mirror assembly: You will need to remove the inner panel on the door. Exactly how to do this varies on the year of your truck. Then, assuming you have the typical power adjusting mirrors, you will need to unplug the harness that comes from the mirror. The mirror itself is usually held onto the door with 3 bolts, have a friend hold the mirror up while you undo the nuts. Once the nuts are off, pull straight out on the mirror to disengage it from the door. Install is the reverse of assembly.
There's a very nice write up here in the 4th Gen DIY section on swapping mirrors that shows the steps.
If it's just the glass you need to replace, I've never done that on a RAM, but if you use the search function of this site I know there are posts present on the subject.
-K