File > Manage > Database
I would HIGHLY recommend at a minimum going through both training series before messing around in the relationship diagram. You can cause irreparable damage to the system if you accidentally disconnect or change anything.
ditto what Mike says!
You can also create some sample database to play in with the Starter Solutions. These are great to open, and check around everywhere.
In addition to the fine trainings, listed by Mike, on this forum are several places to get started with training:
You can also take the file you want to modify, make a copy of it and play with the relationships in the copy until you understand what you are dealing with and work out what changes you want to make. Then, when you are sure of what you want to do, you can start work on the file to be put into actual use. If you screw up the file you are experimenting on, you just toss it and get a new copy.
And that you're new to the FMP environment, in addition to the links provided by Mike and Beverly above, you could do worse than working your way through this => https://www.amazon.com/FileMaker-Pro-14-Missing-Manual/dp/1491917482 If you like learning that way; either by following the tutorial format, or using it as a reference manual to address specific topics. Some individual FMP topics are well presented on YouTube as well.