If you started your course before the 2016/17 academic year you can have a maximum of three tries to pass an assessment: the first try, a second one (in August after summer exams), and then an exceptional final time as an external student the next academic year.
If you started your course in the 2016/17 academic year or afterwards you’ll usually only get two attempts: the first try and one resit. You can only resit assessments you have failed.
If you resit, you can’t get a higher mark than an ordinary pass (either 40 or 50 marks dependant on course).
The number of resits you’re allowed may vary depending on individual situations, so check with your School if you’re unsure.