I was wondering how I update a form as I have tried it before but i'm an absolute beginner. i have this drop down box that has categories and when a category is chose it displays the values of its subcategories in another drop down box.

Depending on how you've implemented you could try one of the following:

1) Include the subcategory drop downs in the code with a display of none and then change it when the proper option is selected, or
2) Use Ajax to fetch the new drop down on the fly (more sophisticated coding), or
3) Create a JS object with the field values and change any necessary drop down using DOM coding (still more complicated that #1).

I personally recommend #1 for ease. #3 for speed. #2 for cool code.