To find your instance please follow this guide: https://wiki.creeper.host/books/minecraft-java-edition/page/finding-your-instances
First, you will need to make sure that the instance you are changing the version on has been stopped (you can find out how to stop your server https://wiki.creeper.host/books/minecraft-java-edition/page/startingrestartingstopping-your-server)
After stopping your instance you can now click on Versions
Versions page click in the
Search Versions textbox at the top of the page and enter the name of the modpack you want to install or update.
When you find the modpack you want to install or update make sure the version you want to install is selected in the dropdown then click the Install button and wait for a notification saying that the modpack has finished installing.