It's super simple at PicMonkey - Just Click Here!

PicMonkey’s photo rotation tool makes it easy to fix photos that are going the wrong way. You can flip images horizontally or vertically, and rotate them right and left.

Open your image in PicMonkey and click Rotate in the Basic Edits tab. A toolbar with four options will appear.
Click the left-right arrow button to flip your image horizontally, and the up-down arrow button to flip your image vertically. The standard clockwise and counterclockwise buttons will rotate your image one-quarter in either direction.
Click Apply.