What fields can SPO modify in a proposal that's en-route (via Data Override)?

[Jennifer Bliss]

1. Activity Type
   
2. Sponsor Deadline Date
   
3. Sponsor Deadline Type
   
4. Mailing Address
   
5. Notice of Opportunity
   
6. Opportunity Title
   
7. Proposal Type
   
8. Project Title
   
9. Attachments (replacements, not new additions)
   

The only way to edit OTHER data/fields or replace an attachment on a proposal that is en-route is to recall the proposal or have a signing official reject it. For more on recall or reject see: What's the difference between recalling a proposal and data override?

[Jennifer Bliss]

How to Data Override a proposal:

When you open a proposal that is pending approval, you will notice that there is a Data Override link located in the Proposal Toolbar:

Click on that link and a pop-up window displays, where you select the field you want to override and the information you want to override it with:

Instructions on how to replace an attachment are in: How do I attach a new file to a proposal in signature authorization routing?
By the way, you can't attach a new file, you can only replace an existing one with a newer version.