Your chain will probably have a quick link, if you can undo that you can remove the chain. The proper tool for undoing this is special
quick link pliers, but you can also undo it with needle nose pliers or even by hitting it with a rock if you have the knack (which I don't, so I'll leave it to others to explain it)
If you're going to replace the chain you could ignore the quick link and just drive a rivet out with a
chain tool. You'll also need a chain tool to cut a new chain to length.
Below are examples. These are Park tools so are at the pricey end of things.
Chain tool:
https://www.wiggle.co.uk/park-tool-chain-tool-ct33
Quick link pliers:
https://www.wiggle.co.uk/park-tool-master-link-pliers-mlp-12