how to change a square photo to a round picture

You can use the Preview.app :

Select Markup from the toolbar, then select : Elliptical Selection.

Hold the Shift key down & resize the Elliptical Selection : using any one of the four blue points indicated below,

this will retain the circular shape for cropping, when you have the size you require, click : Crop :

You will get a dialog box : Convert this document to PNG?

Click : Convert.

Use a third party editor that has that capability as it is not a feature of Photos - If the editor has a Photos extentipon you can edit directly from Photos or you can export the image and use the editor then import the new image to Photos or purchase the external editors for Photos extension from the APP store and use the editor directly from Photos - Photo Editing Extensions for Photos for Mac

LN