The method to achieve what you want is different, depending on whether you are using Storyboard or Timeline mode.
In Storyboard, the actual transition is represented by the blue and orange box/icon highlighted in the screenshot below:
Simply left-click on the box/icon and a menu will open. The first option there is "Cut (no fade)". Selecting this will remove the transition. OR you can choose any other transition shown on that same menu and your new choice will replace the old transition.
In Timeline, the transition icon is the same and looks like this:
In this case right-click on the icon and then proceed as per the Storyboard mode.