Typically a score will begin at bar number 1, but that can be changed — useful if you’re working with an excerpt of a score that needs to correspond to a bar number scheme in other sections of music.
Simply long-press on the bar ruler - a Change Bar Number control will appear, allowing you to type in a new bar number for that bar.
Enter the bar number change in the pop-up keypad. Note that this doesn’t create a new bar number region; rather, it will change all of the bar numbers in your score relative to the new bar number — both before and after the bar in which you’ve applied the change. Depending on the number you enter, this may mean that you will have both negative and positive bar numbers in your score!