Shovels, Spades, Gardening Trowels, & Post Hole Diggers

Sharpen the outside of the digging edge using an angle grinder with a 120 grit flap disc. A metal grinding wheel may be needed for shaping the edge if it is in bad shape.

Sharpening the outside edge will push the shovel's blade twards the hole, and away from the outside (which could make the hole too big).

General Guidelines
If the original angle is different, it is best to match that.