Take a backup before anything risky, and download a copy you can keep.
Take a backup
Take a backup from your server page. Every plan includes backups, and you can also schedule them. Each plan has a set number of backup slots, listed on the pricing page. When the slots are full, delete an old backup first.
Restore a backup
Stop your server. Restoring needs the server to be offline.
Choose the backup you want and restore it.
Start the server again.
Keep your own copy
You can download any backup as a .tar.gz file. Download one before deleting a server, because deleting a server removes it.
Common questions
When should I take a backup?
Before changing the Minecraft version, adding several plugins at once or making big changes to the world.