See below posts
or another way is
- Download the AIA
- Open the AIA with a WinRar or any other software
- Nativate to \src\appinventor\ai_email\app\
- Remove Screen1.scm and Screen1.bky
- Copy Screen2.scm and Screen2.bky to Screen1.scm and Screen1.bky then open the new Screen1.scm and Screen1.bky files using notepad or your favorite text editor and replace all occurences of Screen2 with Screen1
- Save the new AIA
- Upload the AIA to MIT App Inventor
- Done
Finally, I advise you to backup you aia before attempting any changes