When you open the STO client and have entered your log in password you have 3 tabs on the lower side: Holodeck (which is the regular live server), Tribble (the public test server for new patches and builds) and Redshirt (which stands in for Tribble for smaller patches when a new seasonal build is being tested).
Just click on on Tribble before you actually log in and you will automatically receive all the patch data that comes with the new STO build.
Just be warned it will be several Gigabyte if you have bandwidth/hard drive issues/restrictions.
There you can create a new test character, but it would be advisable to copy your existing one from the Holodeck Server to Tribble so you get to test everything up to the endgame content without starting over.
Don't ask me where they hide the copy character button these days.