Answering the question
Assuming you're on Windows here.
XAMPP uses local files on your PC, so you'd be changing permissions of files and folders on your local system - which gives you 2 options.
Personally, I prefer the shell method. Run PowerShell, change directory to the one above the one you're attempting to alter
(example; if you were trying to change the permissions of C:\xampp\htdocs\some-folder, you'd cd to C:\xampp\htdocs)
Then run the chmod command
chmod 0777 some-folder
replacing "some-folder" with, in your case, "public_html"
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The other method is to use Windows file/folder properties, altering the permissions in [directory] -> Properties -> Security -> Permissions
Why we don't do this
Granting read/write/execute to the entire webroot is a massive security hole. Upload and execute anything, brilliant! Goodbye, hard work.
Consider moving whatever needs to write to the webroot into a subfolder and simply making that 0766 instead (owner can read/write/execute, anyone else can r/w), or even the standard 0755 (owner r/w/x, everyone else r/x but not write)