You do not need to pay again to uninstall/install the theme on your licensed store. You can do that however many times you want.
You can disable the theme by choosing a different theme from Administration -> Configuration -> General Settings. If you want to reset your settings to a default state you will need to go Administration -> Nop-Templates -> Themes -> Brooklyn -> Setting and click the Reset Default Settings button on the top right corner of the page. A popup will show asking you which settings you want to reset. You can choose all and click the Reset button on the top right corner of the popup.
