First, you have to remove the door from the tracking. Depending on your door, this is done by either lifting the door up and then 'down', or by removing the molding in the jam on the wheel/roller-side of the pocket itself. So, it may be easy; it may not be easy.
Once you remove the door, mark the rollers and the 'orientation' of the door so that when the pieces and the door all go back 'together', you have everything all in the right place. Then, hopefully, if you know the mfgr. of the door, buy or order parts. If not, take them to a Hardware Store to see what your options are.
Then, reinstall the parts, the door, (and the trim if necessary), and away you go.