(now you know why I gave up growing cactus!) Ow!
You didn't say how big or heavy it is, but
I have one method if you repot (get someone to help you if they are willing) and one method if you don't repot.
To repot:
First, get your new pot ready to go, with gravel, and barely moist cactus mix already in it. Then get newspaper and sturdy tape.
Wear protective gloves once you are all ready- BOTH of you.(suede rose garden gloves have a good long cuff) pick a tool you can tamp with and have it handy.
Take sheets of newspaper and fold them into what will look like a fairly thick 'belt'. Tape lengths together if you must to make a length long enough to wrap around the cactus with enough excess length to grab (as a handle) in your hands, rather like a cinch. Wearing your gloves, wrap the newspaper around and hold securely, then lift the cactus while having someone holding the pot down. Ease it up and out if you can, you don't want to crack the neck of the cactus. If it won't budge, you may have to carefully break the pot with a hammer (if it is clay) or cut it (if it is plastic)
Transfer it to the new pot, and keep holding it in place with the newspaper belt, while your helper fills in any soil needed and tamps it down.
An alternative to potting:
If it looks good and healthy and really could still live in the pot, (no roots exposed on top) fill the top with pea gravel or some other small rounded objects like glass pebbles. That will give weight to the pot and steady the cactus and perhaps it can live happily ever after just as it is.
