If you are the system administrator or say the owner of a file or directory, you have the authority to grant or revoke permission using the chmod command.
You can use '+' symbol to add permissions or '–' symbol to deny permission, along with any of the following letters like: u (user), g (group), o (others), a (all), r (read), w (write) and x (execute).
E.g. chmod go+rw FILE1.TXT will grant read and write access to the file FILE1.TXT, this is assigned to groups and others.