Click on the Startup tab in your control panel, click on the Oxidemod to enable it and make sure the button is blue.
If you have OxideMod enabled under the Startup tab in the control panel the server will automatically update Oxide to latest released version everytime you start up the server.
In other words, if there was to be an update for Oxide, all you have to do is to restart your server through the control panel.
If you wish to use an older version of oxide then go to uMod and download the desired version under history. Disable Oxidemod in the Startup tab and upload the oxide.zip file to the root directory "/home/container/". Right click on the .zip file and choose Unarchive and it will replace the current RustDedicated_Data folder. Now start your server.
Make sure the Oxide version you are uploading is linux supported (Not Windows).