How to switch to the new browser window, which opens after click on the button?

You can switch between windows as below:

// Store the current window handleString winHandleBefore = driver.getWindowHandle();// Perform the click operation that opens new window// Switch to new window openedfor(String winHandle : driver.getWindowHandles()){    driver.switchTo().window(winHandle);}// Perform the actions on new window// Close the new window, if that window no more requireddriver.close();// Switch back to original browser (first window)driver.switchTo().window(winHandleBefore);// Continue with original browser (first window)

Just to add to the content ...

To go back to the main window(default window) .

use driver.switchTo().defaultContent();

This script helps you to switch over from a Parent window to a Child window and back cntrl to Parent window

String parentWindow = driver.getWindowHandle();Set<String> handles =  driver.getWindowHandles();   for(String windowHandle  : handles)       {       if(!windowHandle.equals(parentWindow))          {          driver.switchTo().window(windowHandle);         <!--Perform your operation here for new window-->         driver.close(); //closing child window         driver.switchTo().window(parentWindow); //cntrl to parent window          }       }