Hi, My name is XXXXX XXXXX it is my pleasure to assist you. First off you must be very careful if the mainspring is still wound tight it can pop out at you and cause you harm. So wear safety glasses and be careful. If your new spring has a "C" shaped clamp on it, which comes with it from your supplier to contain the spring. You must disassemble the movement, clamping the springs and releasing the power from both springs. The new spring will fit into the movement with the clamp still attached. After it is reassembled, the springs are wound up until the clamps are free enough to remove. DO NOT attempt this without reading instructions on spring replacement. It isn't a difficult job for most of us, but if you have never taken a clock apart before, you have some homework to do before jumping in. Springs are delivered from the factory with a rust preventative coating that is most definitely NOT a lubricant. With age, this stuff will turn gummy and get as hard as varnish. It must be removed and the spring properly lubricatd before the spring can be inserted. If you do not own a spring winder, It can often be done by hand, but takes quite a bit of hand strength and good leather gloves to accomplish. You best route is to take it to a professional, at least for help in winding and clamping the spring. If you can see the spring, it's probably a loop end spring. In that case, you can get a cheap $8.00 spring winder from Timesavers that will do the job. Part 13473. Thanks Mike

If you need further assistance, please don't hesitate to ask.
PLEASE remember if you're satisfied with my answer rate my answer
"GOT THE JOB DONE" or higher so I will be compensated by JustAnswer
for helping you.There is no additional fee charged for my services.