Watch my short video for a guide to updating using your web hosting control panel. If your web host does not offer this DIY option, ask them to perform the update for you. If they do not offer updates, seriously consider getting a new host.
Note: not all themes and plugins are compatible with newer versions of PHP. This is due to their respective developers not keeping up with the times. If your site does not render or function properly following updating PHP, contact the offending plugin or theme developer. If this is not an option, consider replacing your theme or plugin(s). If all else, simply revert back to earlier versions of PHP until your site is running properly.
PHP is the coding language WordPress is built on, and its version is set at the server-level at your hosting company. The officially recommended version by WordPress is PHP 7.3. (PHP version 7.4 is now out, in beta for testing).