leading or trailing horizontal alignment before iOS 11

Step 1:

View your offending storyboard as source code:

Step 2:

Replace all instances of:

contentHorizontalAlignment="leading"

with:

contentHorizontalAlignment="left"

Step 3:

Replace all instances of:

contentHorizontalAlignment="trailing"

with:

contentHorizontalAlignment="right"

Step 4:

Compile and watch warning disappear.

I find this approach easier when you have a ton of elements that need to be modified. "leading" and "trailing" as 'contentHorizontalAlignment' value types were introduced with iOS 11. iOS 10 doesn't know about "leading" and "trailing" which is the reason for the warning.

In the build log, right before the word "warning", you will see an Interface Builder identifier in the form "xxx-yy-zzz". Copy and paste that into the Xcode search bar, and it will find the "offending" control for you. Click on the search result and it will take you right into the storyboard with the corresponding control selected. The rest of the problem can be resolved using the other answers.

I had this problem with a whole bunch of buttons that I needed left aligned with a little offset. I removed the storyboard alignment and did it like this in viewDidLoad with an array of the needy buttons.

func indentButtons(buttons: [UIButton?]){
    for i in 0..<buttons.count{
        buttons[i]!.contentHorizontalAlignment = .left
        buttons[i]!.titleEdgeInsets = UIEdgeInsetsMake(0, 5, 0, 0)
    }
}