I need to trim 1/4" off the side cabinet ledge, how can i do that so i can mount my microwave?

it's the traditional look-there's cabinets above and on either side of the space aloud for mounting a microwave. the cabinets have a small ledge coming off the sides. i need to trim those edges off so i can fit the microwave in snugly. Is there any other way to do this than using an electric sander, it's taking forever!