To tag a photo that has already been posted:
- Click the photo you want to tag.
- Hover over the photo and click Tag Photo at the bottom.
- Click the person in the photo and start typing their name.
- Select the full name of the person or Page you want to tag when it appears.
- Click Done Tagging.
For More Information Please Refer: https://www.facebook.com/help/227499947267037?helpref=uf_permalink