1.Sign in to blogger
2.Click in you blog name
3.Click on posts
4.On the Top right corner,to the left of the search bar,you would see a drop down box written as "all labels".Click it and select "xyz
5.Now Check each post.(the checkbox is to the left of the title of post)
6 Immeditialy at the top,to the left of publish button,there would be drop down box...click on new label and type "pqr".Thats it