This likely depends on where you are in the application process, so you will have to check with your lender. Canceling a PPP application may be easier than modifying your application. Some lenders, such as Fundera, state that you can cancel or withdraw your PPP application by contacting their customer support.
As far as modifying your PPP application, this depends on a number of factors. A modification may be available in certain cases. For example, borrowers that did not accept all or a portion of their first loan may be able to modify their original applications in the second round of PPP loans. If you need to modify your loan, talk to your lender to find out what options — if any — are available for your specific circumstances.