You do this by moving the pointer over towards the “+” symbol, usually by about 2 or 3 index points. If you’re shooting on a black (or dark) background, you need to do the opposite.
The histogram is the best indicator of good exposure — don’t go by what the picture looks like on the LCD, because it’s always wrong. ALWAYS. It’s only there to give you a general idea of the composition and focus. Don’t bother using it for anything else.