Moodle can work with PHP 5.6 only up to Moodle 3.3, that's the latest version of Moodle you can use without needing to upgrade PHP. Moodle 3.4 onwards requires PHP 7.0 or higher.
So upgrading to Moodle 3.3 (which still has security support) could be an easier short term route whilst you determine the exact steps needed.
To upgrade to Moodle 3.5 you'll need to identify the steps needed to upgrade PHP to 7.0 or above. The exact steps will vary with how your server is configured so step-by-step instructions can't really be given. The steps will include:
- Downloading PHP 7.1 (Thread Safe) and extracting it to a folder alongside your existing PHP folder (e.g. you may have C:\PHP and C:\PHP-71).
- Copying the php.ini from your PHP 5.6 folder to the new folder and searching this file for any explicit path locations, updating these for the new PHP folder).
- Finding the relevant Apache config file (probably httpd.conf) and changing the the location of the PHP DLL file for the new version.
This a rough guide from memory. You'll find better articles on the Web about Apache and PHP on Microsoft Windows. You could separately update PHP (and Apache too if you need to) from getting Moodle updated if this simplifies things.
The most important recommendation if you're unfamiliar with this is to test the update steps on a non-production site before trying to upgrade your site.
There are consultants and support companies out there if that level of assistance is required.