Repair or replace?
Springs, cables and openers are routine repairs and usually worth doing. Replacement makes sense once panels are damaged or the door is old enough that parts are hard to get. Ask for both quoted where it is genuinely a choice.
Can I change the spring myself?
Do not. Torsion springs hold enormous stored energy and are one of the more dangerous things in a house to work on without the right tools and training.
How long does a new door take?
The install is usually a few hours; the lead time on the door is what sets the date, particularly for a colour or size that is not stocked. Ask for both.
Will a new opener work with my door?
Usually, provided the door is balanced and running well. A specialist should check that first, because a new motor on a badly balanced door fails again quickly.