Hi Rajat,
in Grap Builder just click with Left-Mouse-Button on the Legend you want to change and select "Edit" (the last entry) -> "Show Properties".
Then on the right hand side of the screen the Properties appear and in the line "Text" you can write whatever you want for yor Legend.
You can also edit the text there in kind of Font, Font Scale, Color, etc.
I hope this solves your problem.
BR,
Marcus