Our panel makes this one super easy too!
You can choose if you want to keep your existing files, or delete them too (MODPACKS ALWAYS DELETE ALL FILES)
How do I change the modpack installed on my Minecraft Server?
From your control panel, go to Files > Setup > and choose the new Modpack that you want to install - example below:
How do I change my servers Minecraft version?
From your control panel's server page, select the new version from the Server JAR dropdown, like below: