How do you fix a seized mower?

That largely depends on the quality of the engine. If its a cheap/old one its junk. Most often a cheap engine is fatally damaged by a seize. However...

The two main culprits are heat and lack of lubrication. So, allowing the engine to cool and making sure its properly lubed can sometime un-stick a mildly seized engine. (always check a small engine's oil level before each use)

If it is jammed a bit harder spraying a penetrating lubricant (like PB Blaster) into the spark plug hole can help.

Severely seized engines would need to be taken apart and rebuilt.