Replacing a bottom roller on a door that's sitting still, unplugged, with the spring tension released, is within reach for a comfortable DIY owner. Replacing a roller near the top section, or on a door you haven't disconnected from the opener, is not - that's where fingers get caught in track brackets and where a door can drop if a cable slips while you're working.
We'll be honest: if only one or two rollers are bad and the rest of the hardware looks sound, this is sometimes a job you can do yourself with a trip to a hardware store and twenty minutes. We'd rather tell you that than run a truck out for a $15 part. Where it gets harder is older track that's bent or rusted from decades of use - across housing built as far back as 1938, we see plenty of track that needs straightening or swapping alongside the rollers, and that's not a one-person job without the right clamps.