In MacOS you want to edit a screenshot or some image or picture file. What do you do (to not use Preview's view only format)?
To take a screenshot, hold Command, Shift and tap the numeral "5" key. Using all three simultaneously will allow you to capture the screen.
In Preview near the top, click the symbol of an A with a circle around it (near the Search field in the upper-right hand corner). It looks like an anarchy or Acura symbol near the "Search" field; if you cannot find the A symbol with a circle around it, make a mental note of where the picture is saved (e.g., on your Desktop or in Documents). Close the picture, and try opening Preview by itself. From Preview, open the picture. After you click this icon, you should be able to modify the screenshot.
(We tried this in December of 2020.) To browse Mac products, see this link.