Hope the Theory went well yesterday and best of luck for today
When you get there, you could try explaining the situation to the steward and see if anyone in a later slot has cancelled and if you could have their slot. Or try and get there much earlier in case someone has cancelled an earlier slot that you can have instead.
Ask the steward to let the examiner know about the situation, and if you could have a few minutes just to re-focus your thoughts, have a drink, use the toilets or whatever.
If they won't allow you more than a minute or two, ask if you can at least step outside the room for a minute. Just the process of leaving the room, taking a few deep breaths and coming back into it could help you to mentally re-focus on the next exam.
I hope it goes well Annetta - look forward to hearing how you got on
And remember - the examiner is on your side, he/she will WANT you to do well