UPDATE
archlinux-keyring first
then the rest
If there are keys that give an error when updating, you should first check if there is a new archlinux-keyring.

January 2018 it was this key but it can be any other in the future : CEB167EFB5722BD6

If there is no update then you check out this article.

If there is an update availabe for archlinux-keyring, then update that one first with and then the rest.

Read more about this keyring on the wiki.
https://wiki.archlinux.org/index.php/Pacman/Package_signing

A few days later…. when you do now a clean install of ArchMerge 6.3.1 you will get the question to add the missing key from Eli Schwartz to your system and you will be able to update without issues.