If you do not use a version control system (which you should), then make a copy of your existing draft before changing anything.
For your final edit (if there are no story changes left, all gaps closed, and the plot is in shape) read my answer here.
But this does not sound like your final edit. So whatever you do, take a chunk you can easily overview to avoid an overwhelming feeling. "Chunk" varies here from situation and task you want to achieve.
If you only edit typos and grammar, just pick the next two or three pages, correct the errors and pick the next three pages. When editing parts of the story, you have to look at the section, which defines that part. E.g. your ending sucks and it covers the last four chapters, these chapters define your chunk.
Now, it's not easy to overview four chapters. Even more complicated is the fact, that the cause for your sucking end could be somewhere in the first third of your book. If it were easy, it would be called politics.
So make it more easy to overview it by using an (guess what) overview. If you haven't got a short summary of each chapter, then write them now.
Concentrate at one task at a time, get not distracted by other problems you find on your way. Make a note, so you do not forget them.
The most important part of editing: After you made an edit, get the copy of the first draft you made at the beginning and compare your edit with the original part. Read it aloud. Both versions. The biggest problem is editing something to death. When editing, most people use their rational/logical part of their brain. That really can (but need not) ruin your story. Be aware of this trap.
Avoiding this trap needs a lot of experience. I cannot say that I have mastered it. There will be always a doubt. Maybe mastering this step is the difference between an hobbyist and a professional.
Reading aloud helps to find the better text. You hear rumbling passages easily. Silently you read over them.
Sometimes it is so obvious when comparing the text passages, that the new one is better. If in doubt, don't change anything. Or get a second opinion by a test reader.
Some authors say, rewriting stuff is so dangerous, you shouldn't do it all. If that is true for you, you have to figure out yourself. Get experience and become a better writer. I have no better advice, sorry.