Changing the version of PHP that your cPanel account uses is quite easy.
Log into cPanel
Choose Select PHP Version
Note the version of PHP you are using (in case you need to go back)
Select the new version
Click Set as current
Your web site will now be running the newly selected version of PHP and you can test your web sites functionality. If you have issues, you can simply repeat the process and set it back to the version you used to be running (noted in the 3rd step).
NOTE: Changing the PHP version does not change any of your web site's code. It simply changes the version of PHP used to run your site.