Your plum will automatically notify you when your argon is running low, and when it needs replacing. One side of the Plum will need to be free of wine during the swap.
You will need to order a replacement argon tank from https://shop.plum.wine/pages/accessories
From there you will go into Settings > Maintenance > Argon then your Plum will walk you through the steps to replace your argon tank.
There will be a pre-paid USPS return label and a self-sealing box that the tank handle came in to send back your empty argon tank.