To delete your existing word and switch to another modpack please follow these steps:
1. Stop your server.
2. OPTIONAL - Backup any files you might want to keep by copying them to your computer.
3. Log into your server directory with an FTP client (either the built in your panel, or a client like FileZilla) and delete the file named no.update.
4. Choose the new modpack from the Jar/Modpack drop down menu in your panel.
5. OPTIONAL - If you want to enable or disable mods on your modpack this is a good time to do this. You can follow this guide.
6. Save and Start your server.
Please note: Doing this will delete all the files on your server except any backups you have made using the Backup feature of the minecraft panel.