Expiration date of subkeys (retroactive)

[gnupg-users at aschoettler.com]

I have several GnuPG keys which I edited with KGpg.
https://apps.kde.org/de/kgpg/

Unfortunately, the subkeys were not taken into account when setting  
the expiry date.
How can I retroactively edit my expired keys and expire the subkeys?

Important:
I don't want to change the existing expiration date!
So renewing the keys is not an option.