If your site has been set up with rotating captions for images, these captions are kept as text files in the Media Manager.
Go to Media Manager
Open the folder that stores your rotating images (for example "/stories/rotating_images"). Each image file should have a text file with a matching title.
To change the caption, simply change the text in the text file:
Download that file to your computer
Open it the file,change the text, and save it
Upload it back to the Media Manager
The system won’t let you overwrite the file so you’ll need to delete the original in the Media Manager before uploading the revision.
If your image does not already have a matching text file, you can create one in Notepad, and then upload it to the Media Manager.
Use text like this "<center>Place the caption you want people to see here</center>", where the <center> tags describes the placing of the caption.
Save the text file using the same name as the image it should provide the caption for.