Can you use a 13A fuse for a 250w appliance?

If you don't know Ohms law, or how a transformer alters voltage and current in most devices, you probably shouldn't be trying to modify an electrical circuit. Use whatever the manufacturer recommends. Using a smaller fuse may or may not work, but won't hurt anything (the fuse may blow more often). Using a larger fuse than recommended will risk damaging the device, or possibly whoever is using it.