With great difficulty.
A couple of ways
1) Easy one man method
make a double loop out of bailing wire Hook one end over the spring & the other to a ratchet tie down, come along, chain & dogs , block & tackle, whatever you have on hand.
Tie off the mower co it can not move backwards, stretch the spring then lever it onto the peg and cut the bailing wire.
2) He man method
Make a spring hook out of some heavy wire, hook it over the spring and pull backwards using your feet against the mower for support.
In either case the big trick is to remove the deck before you start which takes a lot of load off the spring
