1. Open the ipa with winrar or winzip or any archive program
2. Enter the payload folder and then the .app folder
3. Find the info.plist and extract it
4. Open the plist with wordpad (any text editor should work as well)
5. Find:
[key]MinimumOSVersion[/key]
[string]2.2.1[/string]
6. Rewrite to whatever firmware you want (2.0, 2.1, 2.2)
7. Save it
8. Drag the modified plist back into the ipa/archive
9. Close winrar
10. Your .ipa is ready! Double click to insert to iTunes and Sync...
[Source]