Welcome to the forum. Usually the lens will pop off by using a small flat blade screwdriver on the bottom of the lens. The bulbs are held in by "tension fit" and make contact on a copper finger. Sometimes all you have to do is clean the bug webs or corrosion off to get the lights to work, or else the bulb is burnt out. If none of the marker lights are working, then you probably have a blown 12v fuse. Hope this helps.
Bob