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.
3. Rename the Singular and Plural labels.
Note: Plural label is used to title related lists
4. Remember to "Save" when finished.
Check the below link for renaming tabs and labels.
http://www.infallibletechie.com/2013/08/how-to-rename-standard-fields-labels-in.html
2. Click the "Edit" link next to the name of the child object.
3. Rename the Singular and Plural labels.
Note: Plural label is used to title related lists
4. Remember to "Save" when finished.
Check the below link for renaming tabs and labels.
http://www.infallibletechie.com/2013/08/how-to-rename-standard-fields-labels-in.html