i want to add a new site title and tag line in my website

This is going to be an easy task!

just log into your wordpress site and go to setting right down under tools in the let panel

and click there.It would open a window namley General Settings

right down that type your new site title and tagline title .

go down to page save it.Now refresh your browser ; you have a new site title and tagline .

move your mouse upto title bar and hover there you will see complete site title bar and tagline bar.