Upgrading from 1.2.16 to latest version

I know the answer to my request must be in the forums somewhere but I haven't been able to find it.

I'd like to upgrade from version 1.2.16 to the latest version but I'm not sure how to go about it. Do I need to upgrade every version until I get to 1.3.2?

I've made minor changes to three files:  main.tpl, register.php and search.php

I'm a newbie to this. Is there an relatively simple way to do this?

Many thanks!