Hey Shauna, reading our documentation and in particular the themer’s guide would make a great starting point here: the general principle here is that you should not make changes to code in The Events Calendar (or Events Calendar PRO) plugin files, otherwise those changes will be lost when next you update.
The basic idea is to create an events sub-directory within your theme and place any templates you wish to override in there. So, assuming you wanted to change the details that are visible when a single event is viewed you would very likely be looking at making a change to your custom events/single.php (and possibly the events/single-venue.php) template.
Hope that gets you started 🙂