Install failed: the existing app is signed with a different certificate
INSTALL_FAILED_UPDATE_INCOMPATIBLE
The app you are installing was signed with a different key than the copy already on the device. Android will not treat it as an update.
Why it happens
Every Android app is signed by its developer. An update is only accepted if it carries the same signing identity as the installed version. A build from a different source — a different mirror, a modded build, your own debug build over a Play Store install — has a different key, so the system refuses it rather than letting one developer overwrite another's app.
How to fix it
1. Uninstall the existing app first
This is the only real fix. The app's data goes with it — there is no way to keep it, because keeping data across a signature change is exactly what the check exists to prevent.
2. Check where the file came from
If you expected an update from the same developer and the signature differs, that is worth pausing over. A repackaged APK is signed by whoever repackaged it. Scan it before installing.
3. No installer option can bypass this
Replace existing, Allow downgrade and root all leave the check in place. Anything claiming otherwise is uninstalling for you.
Also shown as
- signatures do not match previously installed version
- INSTALL_FAILED_SHARED_USER_INCOMPATIBLE
- App not installed as package conflicts with an existing package