I cannot edit album art, title, artist in iTunes. It allows me to highlight the text, but will not enable me to delete present text or type anything additional. Please help!!

I want to change the album art and titles for music that I have downloaded elsewhere and then upload them to my iPhone 5s and iPod nano. I select the song I would like to edit and then click "File" and then "Get Info". A box pops up that shows the different text bars for the various categories. I go to highlight and delete the current text in order to type the modified name or album art. Although the it allows me to highlight the text, I am not able to backspace, delete or even type additional text. It allows me to select a new album cover, but wI'll not save and will revert to the original album art. I tried right clicking on the song, choose "Properties" and then was going to uncheck the "Read only" box, but it wasn't marked to vein with.