One of the ways is to upgrade your store to PS version 1.6.1.24 and then activate PHP version 7.1. Once you have done this, upgrade your store to Prestashop version 1.7.4.4 after this update you also update your PHP version to version 7.3 and now update your store to the latest prestashop version (1.7.7.8 at the moment)
This is all possible with the Prestashop one click upgrader. However, sometimes you need to make some additional changes to our change of a specific code.