One of the challenges of sitting an exam is completing it in the time given. We are used to working at our own pace in class and during revision, so being under a time pressure can add to the difficulty of sitting exams.
The only way to improve this is practice! When you are working through past papers or writing essay questions - set yourself a time limit similar to what you would have in an exam.
(If you are not sure, look at a past paper - how many marks is it worth and how many minutes to you have to complete it?
Calculate how many minutes per mark you should have to do it by dividing the total minutes by the total marks. If you then try a 10 mark question - multiply the minutes per mark by 10 to get your time allowed!)
Put away your phone, or any other distractions, then set yourself a time limit to complete the question. Speak to your teacher if you are never getting them finished as there might be ways to be more efficient in how you are answering questions.
It's also an idea to skim through the entire paper first to get an idea of what's in it - then answer the ones you find easy first. You don't have to do them all in order - getting the easier ones out the way will take some pressure off and give you more time to work on what you find more difficult.
Keep an eye on the clock as well - your exam room will have one - it's easy to lose track of time then panic that you won't get finished. If you finish the paper before your time is up, then use the time to recheck calculations, read through answers to make sure there isn't anything that you could add, and to have a guess at anything that you've left blank.