Always backup or take a copy of your server world files before making any large changes to your server!
To get started we need to head over to download the Minecraft 1.17 release from Mojang - follow this link: https://serverjars.com/api/fetchJar/snapshot/1.17-1.17-rc2 and download the latest release of the Minecraft server JAR.
Once you've got the JAR downloaded you need to access your Minecraft servers files, you can easily do this by using a client like FileZilla and connecting via FTP to your hosting provider. (Use this article if you are a ServerFlex customer: https://serverflex.io/help/article/how-do-i-use-ftp-to-access-my-server-files. Other hosting providers will have their own specific FTP login details.)
Using the connection details provided by your hosting provider, enter the "Host", "Username" and "Password" in the Quick Connect bar at the top of FileZIlla.
Now that we're connected to our server you should see a file directory like this, this is the main directory of the Minecraft server. Make sure your server is turned off before proceeding.
Find the "server.jar" file in the list of files, it should be close to the bottom. Now delete the file. We just need to upload the file we just downloaded from the web, and ensure it is named "server.jar". That's it! Turn your server back on and head over to the Console output. You will see a version number read out right after the server starts, this will now say "Minecraft 1.17". Your server has now been updated and you can play the Caves & Cliffs update to Minecraft!
Note: New biomes will not be available in areas you have already explored. New biomes will only be available in newly generated chunks.
If you are not playing Vanilla Minecraft
If your server is not a Vanilla Minecraft server it is likely you are going to have to wait a couple of days, the developers of different modpacks will begin updating their packs to support 1.17, however this will not be instant for the vast majority. Check with your individual provider to see more in-depth update instructions.