Dahlia’s are considered a tender bulb in Minnesota. They should be dug after a frost has killed their foliage. 

Carefully dig around them and lift them from the soil. You can wash them off with the hose.

Cut off any tubers that have been damaged. Cure them in a dry state for 1-3 days.

You can then store them in a semi-moist environment such as a box of wood shavings or in a bag with ventilation. Store at 45°or as cool as possible.

You will find a lot of information and tutorials at the Minnesota Dahlia Society below.

https://extension.umn.edu/how/planting-bulbs-tubers-and-rhizomes

https://www.minnesotadahliasociety.org/tutorials