If it is already in the video you have to put something over it. There is no magic solution.
If you made the lower third and can't figure out how to change the text then you need to start here: Basic AE
Unless you want to spend hours and hours cloning you'll need to create a matching piece of artwork that is opaque where the text is and loose the partial transparency. No other way. No magic button. Like I said before, you have to put something new on top of it. The look of the partial transparency will have to go away.
A shape layer with a gradient or even two would work. Notice how the end fades out so some of the transparency remains. Then just add your new text on top of that.
You should also be careful modifying copyrighted material...