while running: sudo apt-get update in ubuntu 20.04 terminal, I am getting this following error

Err:4 mirrors.hit.edu.cn/CTAN/systems/win32/miktex/setup/deb focal InRelease
  The following signatures were invalid: EXPKEYSIG 277A7293F59E4889 MiKTeX Packager <packager@miktex.org>
Hit:15 download.sublimetext.com apt/stable/ InRelease
Hit:16 mirror.math.princeton.edu/pub/ubuntu focal-backports InRelease
Hit:17 mirror.math.princeton.edu/pub/ubuntu focal-security InRelease
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: mirrors.hit.e…

What should I do to solve it?

@raf The MiKTeX repository you are using has got a problem. If you remove the file for it from /etc/apt/sources.list.d the warnings will go away

@Zanna kindly tell me what I should write on terminal to do it.

How can I tell what bugfixes are available in different releases? I'm specifically looking at evolution-ews, and the changelogs just list a bunch of "New upstream release" (from gnome project, or from debian?)

@raf first you can do ls /etc/apt/sources.list.d to see what's there

@Zanna along with other programs, there exist miktex.list and miktex.list.save

so you can sudo rm /etc/apt/sources.list.d/miktex.list{,.save}

and then run sudo apt update

Now, don't I need to have the miktex.list to keep it updated?

I need to register the GPG key and installation source, right?