If your cursor is on the word you want to replace with the contents of the unnamed register, you can use viwp. v switches to visual mode, iw selects the inner word, and p puts the contents of the register in its place.
If your cursor is on the word you want to replace with the contents of the unnamed register, you can use viwp. v switches to visual mode, iw selects the inner word, and p puts the contents of the register in its place.