Swapping a single roller on a garage door with no torsion spring tension on that side - the two bottom corners, usually - is within reach for a homeowner with the right tool to compress the roller bracket. Pull the old roller, slide in the new one, check that the stem seats fully in the bracket, done.
What we don't recommend doing yourself is replacing rollers up near the top rollers or anywhere the track has already started to bend from a failed roller. Bent track isn't a roller fix, and forcing a new roller into a track that's out of true just wears the replacement out faster, sometimes within weeks.
We also won't pretend every stem length is something you can grab off a shelf locally - some of the 1990s-era hardware in Sherwood uses stem sizes that aren't common stock, which is the honest limitation here: a same-day fix sometimes becomes a two-trip job because the right part has to come from the truck's supplier, not the truck itself.