You may be used to Excel, where a change in the value of an input variable "cascades" throughout the spreadsheet.
In MATLAB, after changing the value of your input variable, you need to reissue your other assignments to get to a new final answer. Up-arrow will let you scroll through your command history, so you don't need to retype the calculations. If you get tired of even doing this, you'll want to capture your calculation into a file (a MATLAB script) that you can run again and again as a single MATLAB command.
