Before starting, you may want to wear some rubber gloves, for hygiene purposes.
You can purchase a new toilet seat from most local hardware store. Each new toilet seat will come with instructions, so follow them to ensure you fit it correctly because they do vary slightly from seat to seat.
Most toilet seats require you to undo two plastic butterfly nuts at the back of the toilet - they should only be hand tight. Turn them anti-clockwise.
Once you’ve taken off the last bolt off, the toilet seat should just simply come off. Then you’ll need to fit the new seat.
Some new toilet seats come already assembled, so make sure you check the instructions.
You’ll need to get the nuts and bolts and slide them into place. Make sure they go through the holes on your toilet and the nuts go back on hand tight. Once you’ve tightened them, make sure the toilet seat’s secure.