Replacing a garage door yourself is possible. Replacing one safely, with a wound torsion spring and a 150-plus pound door over your head, is a different question. We're not going to tell you it can't be done - some homeowners do it - but the failure mode isn't a scratch, it's a spring letting go or a panel coming down off track, and that's an ER visit, not a return trip to the hardware store.
Where DIY makes sense is smaller stuff: swapping weatherstripping, lubricating rollers, tightening loose hardware on a door that's otherwise sound. Where it doesn't is anything involving the spring system or resizing an opening - which, given how many Crofton garages still run original 1988 hardware, is exactly what this job usually involves. If you're set on doing it yourself, at minimum get someone to help lift and hold the door while you work; one person under an unsecured door is how the bad stories start.