"undo a button" is a correct and usable part of a sentence in written English
It can be used to describe the action of fastening or unfastening a button. For example: "I had to undo a button on my shirt to make it more comfortable."
To illustrate that some kinds of compromise have an eternal usefulness, a colleague of Alicia's, Kalinda Sharma (Archie Panjaba), readying herself for a fact-finding mission for a case, good-naturedly undoes a button on her blouse in order to show some cleavage; it's her method for getting a man to give her the information that she's after.
If you have a school tie, then in some laid-back schools you can put them down and undo a top button.
Might as well finish your drink and undo a few buttons while you're there.
Another woman close to Hillary says, "Hillary's the sort of friend who'd undo an extra button on your blouse when you are about to go on a date.
Trowel on a bit more makeup for your selfie, undo a couple of buttons, and watch those Instagram likes double.
Pawing the ground with her stilettos, purring as she undoes a blouse button to welcome prime minister or police chief, Piper proves again what a strong stage actor she is.
Undoing a few buttons on your shirt it always a good idea.
Slytherins are notoriously sexy, the girls undid a few buttons and the guys... well, look at Draco Malfoy!
Once in the vehicle, she said Ghomeshi asked her to undo a couple of her blouse buttons.
To undo a series of changes, click the down arrow next to the undo button, and choose from the series of items you can undo.
The window has also a close button (x button), an add button (+button) and an undo button.
