If the highest PHP version available to you is PHP 7.4 then the latest version of Moodle you can run is Moodle 4.1. See this table of which versions of Moodle work with which versions of PHP.
If you wish to run Moodle 4.2 or later then you must install PHP 8.0, 8.1 or 8.2. If your web server is managed by a hosting provider contact their helpdesk for help upgrading PHP. If you have set up your own web server please provide details of the operating system and its version we will try to help.
Please start a new discussion thread if you need further help with this.