OneBlock is a downloadable map, not a server type. You upload the world folder and point the server at it.
1. Stop the server first. A running server rewrites its world files, so anything you upload while it is on can be overwritten.
2. Upload the map. On the Files page, upload the OneBlock archive and unzip it. You should end up with a folder that contains level.dat and a region folder. If the zip unpacks into a wrapper folder, open it and move the inner folder out, otherwise the server will not recognise it.
3. Keep your old world. Rename your existing world folder to something like world_backup rather than deleting it. It costs nothing to keep and it is the only copy you have.
4. Point the server at the new folder. In server.properties, set level-name to the name of the uploaded folder. The value has to match the folder name exactly, including capitals.
5. Start the server and watch the console. A line reading Preparing level "yourfoldername" confirms it loaded the right world.
Most OneBlock maps need a specific Minecraft version and some need a datapack or plugin that ships with the map. If the map came with instructions about a version, set that version first with the Version Installer — a map built for an older version will often load but behave strangely.
If you need help with this, please submit a ticket.