The Submit button (green with a paper airplane icon) is located in different places depending on whether you're using the web/desktop app or mobile app.
Web and Desktop App
Open a deal. The Submit button appears in the top right action bar.
Mobile App
Open a deal and tap the dropdown arrow next to the deal name at the top. A modal opens with the Submit button (green with paper airplane icon).
Look for the green button with the paper airplane icon—it's the same on both web and mobile.
Why the button might be missing
If the green Submit button doesn't appear at all, see I can't submit deals — troubleshooting for common causes.