how do i modify the paypal order details?
There is a patch in the queue that supports sending itemized information to PayPal, but it was never completed / integrated as far as I know. It gets tricky when you itemize, because any rounding errors in taxes, discounts, etc. triggers a failure to accept payment at PayPal.

In order to do this yourself even without the code being part of the module, you'd need to alter the form used to redirect to PayPal or the API request used to process the payment. Either of these would be possible in a custom module using hook_form_alter() or the relevant alter hook from the PayPal module for the payment method you're actually using.