Is it easy to replace the Air conditioner fan?

There's more than one fan.
When the fan motor is energized, try helping it turn if the fan moves freely and not frozen bearings. If the fan takes off rotating, then it has a bad capacitor. If the fan motor growls, bad bearings and must be replaced.
You can pull the fan section out and take it to an electric motor shop and they fix it, you then reinstall the fan section or blower.

Easy for me, yes. Easy for all, No. If you get the factory part and pay attention the toughest thing is likely getting the blade off without damaging. If you get an aftermarket motor your wire colors change and capacitor size likely does as well. You will then need some experience or know how of what you are doing. Good Luck, buy the motor from the factory if doing yourself.