How do I undo deleting text in notes?

You can shake to undo that deletion and tap to “Undo Typing” and content will reappear.

How do you undo in notes on iPhone without shaking?

To undo text without shaking, just swipe with three fingers to the left side of the screen. An “Undo” prompt will now appear at the top. Keep swiping with three fingers until you have undone the changes. To redo something after undoing it, swipe with three fingers to the right side of the screen.