To change a number or angle continuously select move mode (3.2.1), click on the number or angle and press the + or - key.
Keeping one of these keys pressed lets you produce animations. For example, if a point's coordinates depend on a number k like in P=(2k, k), the point will move along a straight line when k is changed continuously.
With the arrow keys you can move any free object in move mode. The increment is adjustable in the properties dialog (3.1.1).
A point on a line can also be moved along its line using the + or - key.