You need the right tools and then it’s easy
to take off the lock ring and change the cogs.
cog prices vary, just google around for the size you need or ask around.
I have one of these and another similar tool on another bike. They come in handy even at home. You will also need to buy or make a chain whip.

https://www.retro-gression.com/colle...os-trixie-tool