Pour the sweetened condensed milk into the glass jars and place the lids and metal rings on them. Screw the rings on firmly, and place jars on the metal rack in the inner pot.
Pour in water until it just covers the rack, about 2 cups. Make sure the inner pot is flat inside the instant pot base, then put on the lid with the gasket and turn the knob on top to "seal".Set instant pot to high pressure for 35 minutes for lighter caramel, and 40 or even 45 minutes for darker, thicker caramel.
Release the pressure from the instant pot by turning the knob to "vent" when the timer is up. Carefully remove the jars with a hot pad and place on a cooling rack or dry dish towel to cool.Once cool enough to touch, open and use as desired. Cool completely and refrigerate if you want thicker caramel.
Notes
Once cooled, store in an airtight container and refrigerate for up to 2 weeks. NOTE: The caramel thickens as it cools.