After downloading a modpack you may customize it by changing its content.
This can be easily done by following these steps:

  1. Click on the modpack you would like to customize from the 'My Modpacks' tab 
  2. Click the contextual menu (the three dots) and choose 'Profile Options'
  3. Check the box for 'Allow content management for this profile' and click 'Done'
  4. You will now see the 'Add More Content' button appear. Click it to search or browse for new mods, resource packs or maps that you would like to add.
  5. You will now also have the option to toggle mods on or off from the 'Installed Mods' section

Enjoy your customized experience!