That depends on the amount of time you have to make changes and the kinds of changes you want to make. Keep in mind, any and all of the techniques described on the different tabs are worth doing, and might uncover weak spots in your paper that you didn't even know were there!
The very first step (if you have time) is to let your paper sit for at least 24 hours so you can come at it with fresh eyes. If you don't have time, even just a few hours (or twenty minutes!) can make it easier to see things you might have missed before.
If your paper is still pretty rough, and you need to make some revisions that affect the whole paper. This includes changing the organization or structure of the paper, adding missing sections, removing unnecessary sections, or reassessing the assignment to make sure the paper does what it is supposed to do. Check out the Revising page for help!
If your paper isn't in terrible shape, but isn't ready to turn in, you might need to do some editing that mostly address issues at the paragraph level. If your paper has a problem with "flow," this is the section you need. This includes adding and removing parts of paragraphs to avoid repeating yourself while also providing the reader with all the information they need, as well as streamlining ideas, signposting so your ready can better understand your argument or ideas, and making sure all your sources are used appropriately.
If your paper is written and your ideas are all in order, but you struggle with grammar or expressing yourself, you might need to proofread. This includes addressing issues at the sentence level, like grammar, spelling, punctuation, or any of those tiny details that can be hard to spot and seem too stressful after you've just cranked out an entire paper.
Each of these pages offers ideas for where and how to get started making your paper the best it can be. Feel free to do a google search to look for other ideas online - there are literally thousands upon thousands of different methods for improving your writing, and while this website can offer a starting place, you're the best judge for what works for you!