Most of the time, avoid talking about UI elements.

When you need to discuss UI elements, use button instead of command button, option button, or action button. If possible, refer to a button by its label only without using the word button. If you need to use the word button for clarity, button is lowercase. For example, references to an unnamed button such as Maximize may be clearer if you describe it as the Maximize button.

Select a file, and then select OK.
To enlarge a window to fill the entire screen, select the Maximize button.

See also Describing interactions with UI, Formatting text in instructions