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, are all of time in bulk. Select the mentions you want, or all of them and hover the sentiment icon from the top right. Choose the sentiment 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.