How do I adjust the left margin for prototype cells in a UITableView?

Starting from iOS 8 is available the cell property layoutMargins. So the correct way to adjust cell margins is setting this property in your tableView:cellForRowAtIndexPath or in your custom UITableViewCell in this way:

override func awakeFromNib() {
    super.awakeFromNib()

    self.layoutMargins = UIEdgeInsetsZero //or UIEdgeInsetsMake(top, left, bottom, right)
    self.separatorInset = UIEdgeInsetsZero //if you also want to adjust separatorInset
}

I hope this can help someone.

Go to Main.storyboard > select the UITableViewCell > Attributes Inspector. Change Separator dropdown list from Default Insets to Custom Insets. Change the left inset from 15 to 0

You just need to set contentInset property of the table view. You can set value according to your need.

self.tableView.contentInset = UIEdgeInsetsMake(0, -15, 0, 0);

OUTPUT RESULT