You need to open the folder containing the file and (you must be an administrator) Open the file in an editor running as an administrator. OR Right click on the file "C:\Program Files\PostgreSQL\12\scripts\runpsql.bat" -> Properties-> Security. Select the "Users" group, click "Change" and give write access, click apply. Starting with Windows 10, the "Program Files" folder is not writable for regular users and administrators, not in administrator mode. If all this is difficult for you, just copy the file to your desktop, edit and run, it will be the same.
but when I try to save the changes . an error pop out says "you don't have the permission to open this file ,see the owner of the file or an administrator to obtain permission"
even though I went to properties/security/mylaptopaccount and made it full control