Dig out the soil around the base of the fence post. Use a shovel to dig deep enough until you hit the concrete anchor in which the post is set. Remove several inches of soil on the opposite side of the fence post from which it's leaning. This will allow you to push the post back upright into its proper position.
Push the post upright until it is in the correct position. Have someone hold the post so it doesn't move.
Place a 6-inch-long block of 2-by-4-inch wood on the ground next to the post. Position the block of wood approximately 24 to 36 inches from the post. Use a hammer or sledgehammer to drive the block of wood 4 to 5 inches deep into the ground. This is the anchor piece for your brace.
Wedge a 2-by-4-inch board between the fence post and the anchor block in the ground. The brace board typically should be between 36 and 48 inches long, and should hold the fence post in the proper position.
Nail the ends of the brace board to the fence post and the anchor block to prevent it from slipping out of place.