Well you start first by remembering that it's just a job :). A couple of things can help you (depending on time you have)
1- Focus on your preparation, find a friend to do a few mock interviews, in those interviews remember to take your time. Dont rush to answers.
2-Once you have your confidence in the right zone, do a switch : if you were responsible for hiring what would you want the candidate to be good at. With this hat on critique your preparation, and identify gaps to improve on.
If its too late for 1 and 2, dont worry. The core objective was to build your confidence...
Remember you have nothing to lose here, dream jobs do change over time, so just relax and focus on presenting your best self to the interviewer.
If you have presented your best version and there are gaps still? You know what to prepare for in your next opportunity. As long as you see this as a learning exercise, you will be in a good mindspace. If you see this as a test, you will end up with a lot of pressure.
So all the best, hope you present your best self and crack the interview :)