First go to odoo/addons/web/views/webclient_template.xml :
in the head section change 'Odoo' with your new title

 in the title section u ill find :   "title or 'Odoo' "  change 'odoo' with your new title 

then  go to odoo/addons/web/src/webclient/webclient.js:
in this ligne change "odoo" with your new title :

this.title.setParts({ zopenerp: "your new title here " }); // zopenerp is easy to grep