To make it easier to explain, I will create a hollow cylinder with an internal diameter of 50mm, After awhile i want to make the internal diameter bigger let's say 70mm.
Below is a tutorial of how to do this ;)
The hollow cylinder before correcting dimensions
Right click on the fonction that you did use to make the part, in my case extrude fonction ( as you see it's located on the Left panel)
Left click on "Edit sketch "
Double click the left mouse button on the dimension that you need to change
change the value
Quit the sketch
Congratulations you're done,.. final result :)