Here are the steps to Rename the Related List Label
1. From Setup, go to Customize > Tab Names and Labels > Rename Tabs and Labels
2. Click the "Edit" link next to the name of the child object. (Brainshark Activities in this example)
3. Rename the Singular and Plural labels.
(Please note that only the Plural label is used to title related lists)
4. Remember to "Save" when finished.