Start off by finding and tapping the name of the person for whom you want to add a spouse; add a marriage, divorce, annulment, or common law event; remove an incorrect spouse, or edit an event.
When you tap a name, you see the person details screen. At the top is a dark bar that includes the name of the person, the birth and death dates (if available), the 7-character Family Tree ID, the portrait photo (if available), and three dots that give some other options. Directly under that dark bar is another bar. On Android, it's green. On iOS, it's white. To work with marriage information, tap Spouses on that bar under the dark bar.
Depending on the record and what has been added, you see the spouse, the marriage information and the children identified with that spousal relationship. And under that are the Add Child and Add Husband or Wife options. So, if you need to add an additional spouse, it's pretty obvious where to tap to do that. But, if something is wrong: the wrong spouse or problems with the marriage information, or you want to add missing marriage information, or a divorce--then people start scratching their heads and wondering how do to that. So, notice to the right of each bit of info is a pencil icon.
To edit or add marriage information for an existing relationship, tap that pencil to the right of the current marriage information. Now you see options: Wrong Spouse; Edit to the right of the marriage information; Delete Marriage Event; Add Event. Click the one for what you need to do, add the info, add the reason you are confident of what you're doing and it's done.
Incidentally, in my last post about the new look of the iOS app, I talked about messaging. At that time, the option to send a message to a user from the app seemed to not be working. Now it is! Notice that the name of the person who last modified this record is in blue. Tap it and you see any contact information the person has made public as well as an option to Send Message (well it's misspelled on the app as "Send Mesage", but we'll pretend we didn't notice that). The Send Message option sends a message to the user through FamilySearch.