When a layer is selected, its properties appear and are editable in the Layer Properties Inspector which is the sidebar along the right side of the document window.

Change values

You can type values into the fields and press Return to apply the change. You can also drag the label below numeric fields to adjust values.

Use mathematical expressions

Numeric fields accept simple mathematical input such as 10*2 which will be evaluated as 20. You can use operations such as + (addition) - (subtraction) / (division) and * (multiplication)