If it has a large coil spring over the top of the door, you have to remove this under tension, as you can't get to it with the door open!
There will be holes in the end of the shaft that you can peg (a stout nail will do the job) to hold the spring in place. Then you release the cables, so you can move the door. Remove the door. Remove the spring assembly. Handle with care!!
If there are spings either side of the door, IIRC, these are removeable with the door open - this type of door won't immediately then come down on your head - it will wait a few minutes to catch you off guard, and THEN come down on your head.
Whatever the replacement - go electric!