While there are plenty of options (Preview, other special software, etc) Google Drawings is the PERFECT tool for this. Insert your square-shaped image into a new Drawing; be sure to retain the transparent/checkerboard background of the new file. Click on the image, and then right next to the crop icon . . . see it there? It's a tiny triangle! Click on those - always click on those! Tiny triangles and 3 dots: I like to think of them as little treasure chests. In this case, the tiny triangle will reveal the Mask menu. Choose any shape (in this case a circle), and your image will suddenly be cropped (aka MASKED) into the shape of a circle:
To get the most circular circle possible, be sure you crop your original image to the shape of a square first. You may also need to resize the Drawing itself (File > Page setup > Custom > anything that is a square [ex 6 x 6]) so that when you download the new image, it will be as close to circular as you can get.
To download your newly circularized image, just click File > Download as > and BE SURE to choose PNG. This will take advantage of the transparency gifted to you by Drawings and cause your image to be truly circular (or starular or heartular or whatever shape you wantedular).