I ran into a situation where I needed a 5-dollar bill to load it into my smart-card for laundry.
I walked in a restaurant near by and asked the cashier to give me a 5-dollar bill in exchange for five 1-dollar bills.
How would you say that?
"Hi. Could you change these for a five please?"
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.