How to remove a constraint that is already set in Xcode storyboard

Click on a constraint that you want to delete in the Size inspector, it covers with a thin blue border. Even you can select multiple constraints in Size inspector by keep holding command key. The constraint(s) can then be removed/deleted using the backspace.

Or

You can double click on constraint that you want to delete in the Size inspector and it will shows details of constraint in size inspector. Hit Delete button and it will remove/delete the constraint.

Just select the view you want to remove a constraint from and open the size inspector. You should see a list of all the constraints. Select the one you want to delete, and press the delete key.

To help the rookies like me out there, here is how to remove a constraint in Xcode 12.0

0. Click the object that has the constraint you want to remove on the Storyboard.
1. Click the Inspectors button to make sure the sidebar is open.
2. Click the Size Inspector button. See image or look for the right triangle icon
3. Look for the Constraints section, it is a bit down the sidebar.
4. Click the Constraint you want to remove.
5. Hit the delete key.

Open Utilities panel on XCode and select Size Inspector. Here list of all constraints are listed for selected view in storyboard. Now, select constraint(s) you want to delete. Now hit Backspace key on keyboard and selected constraints are deleted.