You can wait until both the Dahlia and Begonia growth dies down. Allow to dry off and remove the soil and dead growth. Don't forget to label the colours and store in a sheltered, frost free environment in dry compost, dry sand or wood shavings. As far as the Dahlias are concerned, if you aren't going to plant anything in the pots, you could move the whole pot to a sheltered, dry, frost free garage or shed, remove the dead growth and cover with compost to overwinter.