In reply to Pradeep Singh.

PHP 7.4 isn't available to the general public yet. It'll also need to be made available by your Web host. Depending on who that is or what your hosting environment looks like, it could be months or longer before you can upgrade your PHP version.

To get ahead, you might want to check out the PHP Compatibility Checker plugin, which will let you know what versions of PHP your themes and plugins are compatible with. Please make sure to read its notes and disclaimers about how the plugin works.