circumcise your hearts: This is a figure of speech. It uses the language of circumcision to describe an inner change.
Circumcision was the physical sign of the covenant that God gave to Abraham (Genesis 17:9–14). Here, however, the command points to a deeper meaning. The people must remove stubbornness and sin from their hearts.
This image includes both inward change and outward obedience to the covenant (Romans 2:28–29).