Some modpacks will automatically update in the Technic Client, and there is no way of downgrading.
This can mean you can no longer connect to your server any longer until it has been updated.
To stop yourself from getting locked out of your server first you want to first say 'no' to client updates when a popup apprears when you launch your game.
The you will want to to set your game client to 'Manually select build' rather than 'Always use recommended builds'
You can do this by
- Click the gear icon below the modpack icon
- Check the 'Manually select build'
- Click save and then start the game
tell them to update and set to 'Manually select build'.