Upgrading and determining current version

I inherited a project a while ago and we're trying to upgrade to PHP 7.

I googled how to find what version I'm using but I don't seem to have a CI_VERSION in the codebase, or a a system directory, there is a core directory but no CodeIgniter.php file.

I'm guessing I'm using a very early version, so what kind of process would be involved in upgrade this to something modern that is compatible with PHP 7?