Hi @Kasey-G , are you using an agile team or the new Boards feature?
If you're using an agile team, go into the team settings. There's a drop down for "Number of days Completed cards stay on the Kanban board".

If you're using the new Boards feature, then Archive the card by clicking the three dot menu on the card. I don't believe there's a way to do this automatically in Boards.