patching a Debian installation package

1. Unpack the deb file in question.

   dpkg-deb -x slack-desktop-LATEST-VERSION-PLATFORM.deb unpack

   dpkg-deb --control slack-desktop-LATEST-VERSION-PLATFORM.deb unpack/DEBIAN

2. Edit the file named control and replace the unsupported library with a supported version/alternative

   libappindicator3-1 -> libayatana-appindicator3-1

3. Repackage the contents of the folder unpack along with the updated control file

   dpkg -b unpack slack-desktop-LATEST-VERSION-PLATFORM-PATCHED.deb

4. Install as usual

   dpkg -i slack-desktop-LATEST-VERSION-PLATFORM-PATCHED.deb