You can swap a roller yourself if the door is down, the springs are relaxed, and you're only replacing the bottom or middle rollers with the hinges still bolted. Pull the old roller stem out of the bracket, slide the new one in, re-pin the hinge. That part is genuinely simple.
Where people get hurt is the top rollers, which sit right next to the torsion spring and cable drum. Pulling a top bracket without locking the door down first can let the whole section drop. If your track has any visible bow or the roller has already chewed a groove into the rail, that's not a DIY fix - a bent track needs to come off and get straightened or replaced, and that's a job with the door under spring tension the whole time. Honest limit: we don't walk people through top-roller swaps over the phone. If you're not comfortable locking out the spring, call it in.