Depending on the study you're taking, you may be able to go back and change or add to a previous answer. Whether this is available is determined by the researcher who created the study, so not all studies will allow it.
Going back to a previous answer
If the back button is available in your study, you'll see it in the top left corner once you've begun:
You can click it if you need to go back to an earlier question to update your response. This can be helpful for fixing mistakes or finishing your thought if you moved forward too quickly.
That said, there are limitations to the types of changes you can make. You can go back to edit:
Any previous question, including ranking, single- and multi-select, linear scale, and open text questions, as long as they are not part of a recorded section.
You cannot go back to edit:
Tasks, such as a five-second test or a prototype test. These are completed once and cannot be redone.
Recorded sections, including follow-up questions on a recorded task and think-aloud survey questions.
If the back button isn't available in your study, you'll only be able to move forward through the test. Make sure you're happy with your answer before continuing to the next question.
Progress bar
As you move through a study, you may see a progress bar at the top of the page. This shows how far through the study you are, so you always know roughly how much is left.
As with the back button, this is dependent on what the researcher selects, so you may not always see it when taking a study.