How do I change server config for everyone in aternos servers 1.12.2 because im tryna change the days between invasions but it stays as default.

You'd have to ask aternos how to access your server files. Don't forget to restart the server and quickly set it to peaceful to get rid of the Daleks.

However, we're releasing a 1.12.2 version soon on curseforge that disables invasions by default. 
However, if you already installed the mod before, it won't work. Only works for fresh installs: https://drive.google.com/file/d/1fPQdkaEZvDOeZs6YajjIOZowWc3tUPte/view?usp=sharing

There should be a file manager that you can use to access the config