Add A Value To Same Field For All Items On Subform At Once

I am using Access 2000 to build a database of car parts, and the cars they are currently installed on. I can assign each item on the "parts" table to a particular car listed on the "cars" table (or to its current location if it is not on a car). I can look at the cars individually, and have a subform that shows the parts currently assigned to that car in a list. That much is great, and is exactly how I want it.

Here is my question:

One of the fields in the "parts" table is "hours." Every time I use a particular car, I would like to increment each of the parts assigned to that car with the hours they were used. Ideally, I would have a form called "Use Vehicle" or similar, and in it I could select the particular car from a dropdown, and a subform would show the particular parts assigned to it. Then, I want to either: a) push a button with a particular time increment (like 0.25 hours) OR b) just enter a value in a box, which would then ADD that number to the "hours" field for every part in the subform.