Websites and web apps often ask permission to perform tasks. Once you grant or block access, the site won't ask again. So how do you change those settings?
Usually, your web browser's address bar is the quickest way. Next to the address you will see a sheet of paper or a lock icon. Click or tap on this (or maybe long press) and you should get a menu to let you change your settings.
Another good article on changing your permissions can be found on HowToGeek.com
For Android web apps, you can long press on the application's icon then select app info, or go into settings / applications and locate the application in the list. Once there, you can alter the settings.