Hello there, @ellie25 ...
Without being able to see how the instructor/lecturer set up this document within your course, it may vary on what options are available to you. If the file appears to just be linked in the course as "Read Only" (meaning you can only view/read it without editing), you would need to download that file to your local computer. Then, assuming you have a word processor installed on your computer such as Microsoft Word, you would open up that saved file, make any edits, and then if necessary, submit it back to your instructor.
Another option that your instructor might have set up (but you would need to confirm with the instructor) is what's called either a Google Cloud or Microsoft Office 365 cloud assignment. Essentially, what happens here is that students are able to view a document online, and then students connect their own Google or Microsoft account within the course. The cloud assignment then creates a copy of the document shared by your instructor and saves that copy in your own Google or Microsoft account. You can then edit that file right online in Google or Office 365 without disturbing the original document that was shared by your instructor. You can then save/submit you edited version of that file back to your instructor for grading.
Ultimately, it would be a good idea to get in contact with your instructor to see exactly what the requirements are for the document and what he/she recommends for that particular assignment. I hope this will help a bit. Take care...be well.
