This error is raised when the signatures of the previous and new versions of the application are different.
Please read more about the application signatures here: https://developer.android.com/studio/publish/app-signing
There are two possible solutions of this issue.
1. Sign the app with the correct keys
2. Force uninstalling the previous version and install a new one (all app data will be lost!). Read here how to do this: https://qa.h-mdm.com/1954/
If you care about the app data and would still like to update the application without removing the previous version, you need to address this issue to your development team.
There are two general reasons why the app could be signed by wrong keys:
1. The developer has released a debug version by mistake, because the debug version is by default signed by debug keys which are different from release keys.
2. The development team has been changed and the keys are lost (that's the worst and unrecoverable case!)
Please ell the following to the client's development team. The signature of the new version is different from the previous one, and I cannot update. Please could you fix the issue by signing the new version by the same keys as the previous one.
After you get a correct version, just upload the new version in the "Applications" tab. When the control panel asks you whether you'd like to add a new application or a new version, answer "New version". The wrong version will be replaced by the right one.