Say I have a view whose width is 1/2 of the superview in portrait, and 1/3 of the superview in landscape, I thought I could set different multipliers for that constraint, but size class doesn't have this feature.

Then I found a solution in the following post, which says I have to disable and enable constraints for different size classes.

Changing the multiplier of a constraint based on size class

I don't know how to do it. I only find a way of clearing all the constraints. But I don't want to rebuild all the constraints again. I just want change for the one constraint. If I delete that constraint in my current size class, it is also deleted in other size classes. Is there any way I can disable that constraint just for one specific size class?

Use isActive property.

As documentation says

The active state of the constraint. You can activate or deactivate a
constraint by changing this property. Note that only active
constraints affect the calculated layout. If you try to activate a
constraint whose items have no common ancestor, an exception is
thrown. For newly created constraints, the active property is NO by
default. Activating or deactivating the constraint calls
addConstraint: and removeConstraint: on the view that is the closest
common ancestor of the items managed by this constraint. Use this
property instead of calling addConstraint: or removeConstraint:
directly

This is how you delete a particular constraint in a particular size class

1.) select the specific size class you need

2.) Click on the view or uielement whose constraints you want to remove/add.

3.) All the constraints of that view or uielement is shown in the right side Attribute Inspector.

4.) Select the particular constraint you want to remove , which sets a blue border on that constraint.

5.) Then press Backspace.

This will delete it for that particular size class only.

This constraint now appears in grey on for that
View Controller.