How to fix apt update EXPKEYSIG 1DB7590E83C8F643

Problem:

When running sudo apt update you see the following error message:

Err:14 https://ocean.surfshark.com/debian stretch InRelease
  The following signatures were invalid: EXPKEYSIG 1DB7590E83C8F643 Surfshark package maintainer <[email protected]>

Solution:

Download the updated Surfshark release package (which contains the updated key from the repository) from the Surfshark website and install the package e.g. using

sudo dpkg -i ~/Downloads/surfshark-release_1.0.0-2_amd64.deb

After that, the error should be fixed and you can run

sudo apt update

again.