we must edit the master pages.
1) To access the master pages go to Admin > Layout > Theme designer
and click on the theme name
2) Click on the masters tab and then the common.masters file
3) Click the edit button in the top right corner (pencil and paper icon)
4) Near the top of the masters page you will find the tag
5) Edit the text between the title tags to reflect your desired text and save
6) You will also need to edit the headers in commerce.master,
catalogpartdetail.master, commercecart.master, commonlogin.master,
catalog.master and support.master to point to you newly modified
7) This is achieved by changing the line
MasterPageFile=”common.master” in all of the files listed above.
8) Note: Partdetail automatically will add the Item No and Item Name
of the currently viewed product to the title set in the common.master
file. This helps with search indexing of your Items.