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.
sudo pacman -Su archlinux-keyring
Read more about this keyring on the wiki.
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.