Trim obscures outlet on mantel

There is an electrical outlet on the mantel above my fireplace where trim was installed in a way that partially blocks the outlet. I want to put a lamp on the mantel, but the plug won't go all the way in. Someone just cut the outlet cover in half to make it fit, but that doesn't let plugs go in all the way. What tool or method would you recommend I use to cut part of this trim away so I can use the outlet safely?