Looking for help; How to upgrade from php5.2 to current version

Hi, Please can you offer some advise or help. My father and I had a website running php 5.2 however this website has now closed down and php 5.2 is no longer supported via the hosting package. We brought a new domain to run our php code before we knew php 5.2 is no longer supported. The php code was written for us by one of my father's ex-work colleagues and unfortunately have lost contact, so we are a stuck at the moment! We would like advise/help on how we go about upgrading our php code to the latest version of php. We don't know if the code works beyond our first page as soon as we go to log in we get an error message. Look forward to hear back from you. Kind regards, Richard Tebbutt