I struggled with this for some time as well.
In order to delete the bottom constraint, I made sure that Stack View was selected in the directory hierarchy across the top o the window (or you could select Stack View in the Document Outline at the left). There is a very small constraint beam at the bottom of the Stack view below the UIImageView which can be selected and then deleted. This will remove one of the constraints but I can’t remember which one.
But if you try to drag the bottom of the stack up, it keeps reverting to sticking to the bottom.
The trick is to drag the top of the stack up, holding above Item, and let go. It will stay up and appear like the view shown on page 262.
Now when you insert the UIToolBar, it will be above the bottom of the stack, rather than underlapping it.
When you drop the UIToolBar, make sure it is well off centre and hasn’t stuck to either side, so that the values in the left and right constraints as shown on page 263 can be altered to zero, and thus count towards your 5 constraints to be added.