I prefer to use the words 'Share on Tweeter' rather than 'Tweet on Tweeter'.
This is how it is done:
1. Go to Plugins on WordPress Dashboard.
2. Click on Installed Plugins. Then click Activate Plugin.
3. Scroll down to Mashshare Share Buttons.
4. Click on Edit.
5. Under the Plugin Files on the right side of the screen, click on mashsharer/class.mashsharer.php
6. Scroll down until you see this section of the code:
7. You can see the two hyperlink texts 'Share on Facebook' and 'Tweet on Tweeter' there.
8. If you want to change the hyperlink texts, do it here as highlighted in red above.
9. Then click Update File button.