On the app dashboard, tap the Stash tile, pick the Stash you want to close, hit "Manage," then click "Close." If you're a hundred percent sure you want to close the Stash, choose "Yes, close the stash."
For Solo Stashes, your Total Stash Balance (including interest earned) will go to your Tonik Account in a single transaction.
For Group Stashes, the Total Stash Balance (including interest earned) will go to the Group Stash Owner's Tonik Account in a single transaction.
Always remember that the group's Total Stash Balance belongs to the owner of the Group Stash. They're the only ones who have control over the fund and can close or withdraw from it at any time.