You can change the Apple Developer ID that Xcode has associated using Mac OS X’s Keychain Access app:

1. Quit Xcode
2. Open Applications > Utilities > Keychain Access
3. Locate the Keychain entry named “daw2.apple.com ([your Apple ID])” of kind Internet password
4. Delete this Keychain entry and retry the download in Xcode by re-entering your credentials, or double click on the entry and update the Account and Password fields
5. Open Xcode and retry the content download