By default in the Graph Editor, you can simply click and drag to move a selected key on a curve.
If you prefer to disable this single-click editing workflow, you can disable the Left Mouse Button Alters Selections preference (Window > Settings/Preferences > Preferences) and use the Move Nearest Picked Key Tool to explicitly move keys with the middle mouse button. (Disable Left Mouse Button Alters Selections in the under the Graph Editor heading in your Animation preferences.)
To move keys along an animation curve using the Move Nearest Picked Key tool
See Select Keyframe Tool.