frem
Are you using a Windows PC?
You might find it easier to use Cyberduck if you're not sure about commands
Cyberduck | Libre FTP, SFTP, WebDAV, S3, Backblaze B2 & OpenStack Swift browser for Mac and Windows
Cyberduck is a Windows Explorer type interface and gives you a 'directory' based interface;
it is VERY intuitive to use, especially for those not so comfortable working with Linux commands.
Select the SSH protocol to your box's local IP address and enter the root/libreelec access
once you connect the first thing to do is click on the 'view' tab and select 'show hidden files
You need to do that to expose the 'update folder' and the .kodi folder
Then you can navigate to the Addons27.db file (>kodi -> userdata -> database -> and just highlight the file & delete it (as you would if you were in a regular Windows Explorer directory)
Then navigate back and open the 'Update' folder, and use the 'upload' icon at the top of the screen & select the image file from your windows directory
You should see it in the upload folder if you navigate 'back' to the /storage list.
Even if you inadvertently upload it to /storage, you can 'move' it into the update folder by just dragging & dropping within the window!
You can either go back to the module & reboot from there, or simply from Cyberduck - from 'go' menu select 'send command' and type 'reboot' and click 'send'
For the second part of your question, placing the new image file in the update folder and rebooting will execute the actual upgrade.
Once it has completed the upgrade it will reboot itself and if you were to reconnect by SSH into the box, you would find the update folder is now empty.
You cannot revert backwards once you have upgraded so you may want to save your original config if necessary.
In actuality you won't see much difference I suspect operationally other than the K17 differences, so if you have already experienced K17 on your other box, you should probably have no need to revert.
Footnote - even though you deleted that file, your addons from 16 will all migrate.