Touch the Tools icon at the upper right...
..then from the menu select Header and Footer. The Header and Footer editing window will be displayed:
To enter text, touch the Header editing field to display the blinking cursor and keyboard and then type in your text.
To insert a page number, date, scene number or other data field, touch the Header editing field, hold for a moment and let go; the Insert menu appears. Touch Insert and from there you can choose what data field(s) to insert.
Options to show the Header and show the Header on the first page are here as well.
A footer may be set up, but footers are not usually used in scripts.
Text and data fields can be removed from the Header or the Footer by touching the screen just to the right of the field and then using the Backspace key on the keyboard.