How do I remove a top of a Cactus that fell over and bent this fall?

Just chop it off.

Depending on what type of cactus cutting off the top may kill the plant. If it a Christmas cactus then snip away then stick the cuttings in some dirt and start new plants. If your cactus is a type with needles like a barrel cactus it could kill the plant.

Also remember to wear gloves to protect yourself from getting pricked.

What kind of cactus is it? What is the aprox size of the portion that fell off?

https/world of succulents.com/pachycereus. I am having trouble with the link but World of succulents.com has great care instruction. You can apparently cot off branches let the ends dry then pot it . Check out the site I think the top should heal but I can’t be absolutely sure. But the top that you cut should start roots when it’s end drys.