How do I Regrout shower pan?

https://www.familyhandyman.com/tiling/grouting/how-to-regrout-bathroom-tile-fixing-bathroom-walls/view-all/

https://www.popularmechanics.com/home/interior-projects/how-to/a6631/tips-and-tools-to-regrout-your-bathroom/

You mean the fllor tile over the pan? Is it leaking? More info needed. A pan doesn't have grout.

For the shower pan only, where the grout is already crumbling and falling out, you need some strong grout or lots of silicone caulk. Turn the water off to the shower, or bathroom, or house. Use a small chisel or large flathead screwdriver and a hammer to knock out the old grout. Use a rag to make sure you are leaving a clean surface. Mix the grout according to directions. Fill the space between the shower pan and the wall. If all of it is dropping inside the wall, add more grout to the mixture to make it thicker. You could also use strips of muslin or cheesecloth to help hold the grout. Whether you use grout or silicone caulk, which is waterproof but costs more, use your finger to make a smooth edge and top, cleaning off any dripping grout or caulk. Do the smoothing (or adding extra), a couple off times, but don’t leave sharp edges. Let the substance dry AT LEAST 24 hours before it gets wet. If you don’t, the grout will turn sandy and start crumbling. Now, there are more professional ways of regrouting, but this is one way an amatuer can do it. Best wishes ☺️