IMO, if it’s a field in the user data, you should be able to update it via one of the two API calls for that. 

Potential Use Cases: 

• Bulk updates of Custom User Titles (ex. for Staff or Customers or a large group of a specific user segment)
• Ad hoc updates upon user request (ex. you have a staff rank that calls everyone {company} employee, but you want an opt-in option for Product, Support, etc to share their specific titles (that’s relevant, helpful context when they engage!). You could easily do a form > API automation to save time, rather than community managers as a bottle neck
• (my use case) an AND with ranks
  • I wanted to do a special rank for last week’s leaderboard winner, showcasing them with a rank icon and rank that visually calls them out
  • But I also wanted to keep the context of their Rank (certification, experience level, super user status, etc)
  • If I had the option via API I could have updated their custom user to title to override the rank title so that it’d always be “{Rank} + Last Week’s Top Contributor” (and then I’d toggle it off after a week via automation as well)