Assuming that you have attempted all questions, there are two main things that you can do during the remaining exam time. First, there is the standard process of reviewing your answers to every question. This includes reading each question to ensure that you have correctly understood the questions and that your answers are consistent with the demands of the questions. In other words, you should first scan and proofread the entire paper.
The second element is essentially an extension of the first because it involves applying the same approach but to specific questions. Students typically skip some questions during an exam when they are not immediately aware of the approach required to solve/respond. At the same time, such questions may be more difficult, meaning that students may spend more time on those individual questions compared to the rest of the questions in the exam. The higher difficulty of the questions can sometimes be associated with a greater chance of error. Because of this, it is important to place a greater emphasis on questions that you may have struggled with when reviewing your answers (for example, diligently reviewing each step of your working). Doing so will increase your chances of identifying any errors that you made (if applicable) and address them during the remaining time.
I hope this helps!