1, when downloading, you must use your own account to download from ITUNES, and don't download other people's good software from the Internet. Otherwise, the signer will be invalid.

2. If the updated software or purchased software prompts "invalid signature" and "resource has been modified", please manually "delete" the software under the path "My Documents \ My Music \ iTunes \ iTunes Media \ Mobile Applications" to the recycle bin. Then "cut" back to the path of My Documents \ My Music \ iTunes \ iTunes Media \ Mobile Applications in the recycle bin, and then "double-click".

This way, you can install or update by syncing with iTunes again (if not, please try several times).

3. This phenomenon may also be because the downloaded file is wrong, so I went to official website to download it again.