Android Question PLaystore error :needs to be signed with the same signature

I stored an App on the Playstore compiled with B4A installed on Xp .
Yesterday I moved all my files to a Win7 environment and tried to do
an Update APK on the Playstore.

I got the follwoing error:
The updated APK needs to be signed with the same signature as the current version.

I have two question :
- were can I find my old signature
- how can I change my Win7 signature to this old signature .