Release file is expired - E: Release file for is expired (invalid since …)

I’m getting for a few days when doing apt-get update that the release file is expired:

“E: Release file for is expired (invalid since …). Updates for this repository will not be applied.”

What is this?

Whonix 14 (Debian stretch based) is deprecated. Please upgrade to Whnix 15 (Debian buster based).

Thanks for the reply.

Is there any way to easily upgrade?

How do I safely move files from 14 to 15? Between Workstations?

Easy or not…

You tell me.

Lengthy. Nontrivial.