Hi,
Please follow the below steps.
1. Click on Extract files in file menu.
2. Save your form in a folder.
3. Then go to the location where you have saved your form and open it.
4. you have manifest in your saved folder.
5. Right click on manifest and click on design.
Then you can do the changes in that.
Note: It is available in InfoPath 2003.
If you are using InfoPath 2007 then click on Save as Source Files in same location.