Easy, here's how:

Go to the post, e.g. in your profile and open it. In the lower right corner there are three dots.

Tap them and then tap "Edit", edit and save. 

You may need to refresh to see your edits, but rest assured, they are there.  

Want to edit a comment you made on someone's post? 

- Go to the post and press "Comment"

- Press the tree dots next to your comment and choose "Edit" - there you go!