Hello you are probably looking for 'equation' Feature of solidworks.
I can give you an easy example below:
Lets assume you want a cylinder which has an height of 3/4 th of its Diameter.
And you want to Change only the Diameter value and let the SW Change the rest.
Create your first sketch of the circle with a defined Dimension for the circle, 20 mm in this case.
SW saves this value as 'D1@Sketch1' which you haven't probably notice till now.
Under the Boss-extrude Feature, instead of giving a specific value you can put an equation (just like Excel sheet).
For our case it is 'D1@Sketch1*3/4'.
This equation will automativcally Change the Boss-extrude value everytime you Change the Diameter in step 1
If you want to Change the equation or any relationship between dimensions, just go under the 'Equation' tab in SW.
You can Change, update or delete the relationship anytime you want from here.