Must I start over if I change a dimension in Fusion 360 As a newbie, I have done some Fusion 360 tutorials and recognize I made a mistake in the initial dimensions.

Aside from starting the entire sketch over again, can I just modify the (beginning) rectangle dimension?
If you enabled the history, you can easily go back in time and change the dimension in the sketch and then go back where you were. It would be even more elegant if you do not use fixed valued dimensions in the sketch, you can use parameters you define yourself and assign them to certain dimensions to create parametric designs. These values can be changed at any time to see your final design adapt to those changed parameters.