I have attempted to install an apk to my phone with an app that I already have installed. I have changed the version number however the issue is clearly down to the original signature of the app.
The app itself was originally developed by another company so I do not have the original signature for this? Do I need the original keystore file to allow for this?
Also it is unlikely the external company will still have the original keystore file - if this is the case & I cannot use the original key what is the next step from this?
Obviously i'd like users of the app to be able to 'upgrade' the app through Google Play without directly uninstalling the app manually themselves first.
Can anyone provide a solution ideally without the original keystore file?
You can't upgrade with new keystore as @orip said.
You could ask your developer to send you keystore file. Otherwise you will have to change package name, sign with a new keystore and upload as a new application.