What failed
Compare the version in GB WhatsApp Pro → Settings → About with the version you just downloaded. Trust the About screen, not the filename. “Problem parsing the package” or “package appears to be invalid” usually means the file was cut off. Delete it and download again. Do not rename .apk to something else.
A signature conflict means this APK is not an update to the copy on the phone. It is a different package. Android will not overwrite it. Uninstalling is the only way to switch, and uninstalling removes local chats unless you already have a backup.
Before you uninstall the version that still opens
- Leave at least 1 GB free so the installer can unpack the file.
- Charge the phone or keep it plugged in. A shutdown halfway through can leave a half-installed app.
- Back up while the old version still opens. Do this before you remove it to clear a signature clash.
If the in-app backup button errors or never finishes, stop the uninstall. Fix that first with a GB WhatsApp backup that fails or never finishes. A failed update does not delete chats by itself. Removing the app does.
Expired screen is not an install error
If the app opens and then says the version is too old, the APK you have is already installed. You do not need to fight the installer. That screen is an expired GB WhatsApp version that will not connect. Update from a file you can match by size, after the backup above is done.