This method is if everything fails (quite tedious).... it worked for me every time so far
You need:
- Unlocked phone
- TouchXperience (Windows Phone Device manager) or advanced explorer (So that you can transfer files from PC to Phone)
1. Make a copy of the .xap file (just in case u screw it up)
2. Open the .xap file with winrar
3. Extract everything into a New folder
3. Find the file "WMAppManifest.xml" - open it and check the "productID", record this number down
3. By using winrar, delete files inside the .xap file until you have around 20mb left of the .xap file (Make note of what you deleted it, i usually just delete the content folder)
4. Deploy the modified .xap file
5. Now by using Touchexperience, transfer the files u deleted into the following destination in ur phone: (Just in case u forget, you have already extracted all the files previously, so find the deleted files from there)
Applications\Install\"Product ID"\Install
(make sure the folder structures remain the same as it was in the .xap file)
6. And you are done
I hope i was clear with this!!~~