There are varying degrees of complexity. If there are children, you have to delete or attach them to someone first so that the unwanted spouse does not have others attached.
I next go to the Person View/Page of the People mode and delete the marriage event fact so that the marriage no longer ties the couple together.
Then I go to the Family view and delete the spouse.
If ever I get it messed up and have an Unknown as spouse anyway, I rename the unknown as Poop. Then I merge Poop with the legitimate spouse in order to have only one and get rid of the name Poop.
Why Poop? Because I may have many Unknowns, but only one Poop...so I can easily find the Unknown I am trying to get rid of. If ever I find the name Poop left amongst my names I know I still have work to do to finish.
I think that if the spouse you want to eliminate has children that really belong to the spouse you are keeping, then you could just merge the two spouses while choosing to keep the data for the correct one and discarding the data for the one you wish to eliminate BUT I HAVE NOT TRIED IT.