You can easily remove (disconnect) connected devices from your MysteriumVPN web dashboard. Just go to the dashboard and select the "My devices" section:
To remove the device, just click the "Remove" button. Please keep in mind that the total number (slots) of devices you can connect to your account depends on the package plan you originally purchased.
Basic Plan allows to connect 6 devices at the same time
Plus Plan allows to connect 10 devices at the same time
Pro Plan allows to connect 15 devices at the same time