I'm writing a paper in collaboration with another author. However this author is writing too much of the paper, including parts that I said I was going to write, and the way he wrote it is not good in my opinion, or not up to the standard we are aiming for. How can I replace his parts with my own without creating conflict?
Constructive criticism is the best way to go in these cases, unless you are the primary owner of the paper, it's gonna be hard to bulldoze your opinions. Give comments and suggestions on things that aren't addressed or sections where it isn't clear. For the most part, these papers go through a lot of changes so it's normal to rewrite the whole paper incrementally.
Another approach is that you can have your supervisor/advisor do a review and then ask them to make the changes. If the work produced by your collaborator is bad then it's more complicated and your solutions are heavily dependent on how much of a working relationship you want to keep intact with this person.