Modify the submitted manuscript to journal
I think the best course is just to wait for the reviewer reports. Probably they will suggest some changes. Make additional changes at that time and submit them with the revised manuscript. Otherwise it will just be confusing, even chaotic, for the editor and reviewers. Let the process run for a bit. 

I'm assuming that the changes you want to make wouldn't make the difference between acceptance (with revision) and rejection. If you get rejected, revise and submit elsewhere. 

But don't do things likely to confuse the editor.