What should I do in a speaking exam if I don't know a word?

Don't freak out - if there's a specific word you want to say but can't remember/ don't know what it is in Spanish, think of a way you can talk around it using the words you do know (that way, the teacher who is assessing you can guess what you're trying to say and help you out a bit). And try to avoid speaking English if you can.
For example, if you wanted to say 'manzana' (apple) but couldn't remember the word, you could talk around it by saying something like 'no puedo recordar la palabra, pero es una fruta y si comes esta fruta cada día, no tendrás que ir al medico' (I can't remember the word, but it's a fruit and if you eat this fruit every day, you won't have to go to the doctor).