Yes, you can change the sentiment for a mention. If you believe that the sentiment classification has been done inaccurately, you can manually change the sentiment, just like in the screenshot below.
You can also change the sentiment for several mentions, in bulk. Select the mentions you want ( or all of them from the top bar) and chose the sentiment icon you want to apply to the whole section of selected mentions. This way, all the selected mentions will have the sentiment according to the one you chose.
