You can change the number of undos you can do in the General settings, change the value next to Maximum number of undos.
An additional side bar is displayed with the options of the selected tool.
When you are using the brush or eraser tool this is where you adjust the size and opacity of the brush.
When you are using the selection or transformation tool, you have access to different options of selection and transformation.
You can choose to have the side bar on the right or on the left of the screen, to do so, in the General settings, switch the Flip side bar position button.