We have built a application with using standard Lightning component(ex: Overdue Tasks, Recent Records, upcoming events etc...) which has been provided by salesforce. But I would like to know is it possible to update or change UI style with using stylesheet for the standard component ? If yes how ? Kindly do let me know ASAP.
You technically could wrap the components in a custom component and alter the styling from the styling file, however this is not really a good idea, since these components are subject to change and your component styling could break in an upcoming release.
It is always recommended to create your own custom component with your classes in conjunction witht he SLDS in order to properly style your components.