It sounds like you might not be logged in. Whenever you are logged in, you see an [x] next to each of your tags; just click it to dete that tag and you can then create new ones.
To delete one of your images, go to your MyAccount page (by clicking on your user name on the top right hand side) and then click on the photos tab. Each of your photos will have the text link 'Delete' underneath it.
To delete tags you have added you need to go to the page for the business you added them to. Does that make sense?
To delete tags that are already in the system (not from you), you can submit a correction and make a note of them -- or email support at yellowbot.com and we can try to help.