Absurdly simple question- delete tag and replace

Absurdly simple question, that I think can be answered in 30 seconds and has defeated me for 30 minutes.

How to replace a tag (which is populated by many different text values over many files) with another tag?

For example, a list of songs has "Rock", "Alternative Rock", "Pop" as GENRE tags.

Wish to have all those songs have "Rock" as the GENRE tag.

How to do so?

Thank you!

Select the files where you want to modify the genre.
Enter the preferred text in the input box for genre in the tag panel on the left.
Press Ctrl-S to save the changes.

Sigh....thank you. That was, as suspected, very easy...