You only need to do this if you're prompted by the launcher, with an error similar to this:

To fix your hosts file:

1. Open Notepad as administrator (right click, Run as administrator)
2. Click File -> Open %systemroot%\System32\drivers\etc\hosts (you can copy and paste that into the "File name" box)
3. Delete the lines that don't start with a #, then save the file (File -> Save)
4. After that, just login as normal.

If the steps above don't fix your issue, drop by our Discord server for help: https://discord.gg/technic, join the #tech-help channel.