You can go to your cPanel and there under the Software tab you will find MultiPHP Manager icon,
Click it, then choose the directory you want to run PHP 7 or 5 on, usually public_html. Choose PHP7 or 5 and save.
You should now see "public-html (PHP70)" in bold on the directory.folder tree.
That's it, your site is now running on PHP 7. Check your site to make sure there are no issues. If you have issues, narrow it down to which extension(s) are causing the problems and contact the developer. In the meantime, you can change the PHP version back to 5.6 with a couple of clicks in cPanel.