Hi Nagarajan, 

To Change a label in chat Form, Please follow the following steps:
1. From Setup, in the Quick Find box, enter Rename Tabs and Labels, and then select Rename Tabs and Labels.
2. Set your primary deployment language from the Select Language dropdown list.
3. To rename a tab, select Edit next to the Contacts, Leads, or Cases based on your pre-chat objective. Select Reset to revert to the original name.
4. Enter the singular and plural forms of the new tab name. Also, if applicable for the language, select Starts with a vowel sound for labels that begin with a vowel to ensure the proper article (such as a or an). 5. Then select Next. When you rename a tab or an object, you can’t use the name of another standard tab, custom object, external object, or custom tab.
6. Enter your labels for the standard field labels. If necessary, enter both a singular and plural form for each label. Select Starts with a vowel sound for labels that begin with a vowel. Then save your work.

IMPORTANT If you’re customizing the First and Last Name fields, complete each label separately. The First Name and Last Name labels (defined by Contacts and Leads in the Rename Tabs and Labels Setup) are used by the pre-chat form.