Go into Manage images, double click the image.

In your image editing box you just go to the attributes tab. At the bottom of that page you can change the license type. Just change it from RM to RF. Pretty easy actually. But, you can't change it back.

I contacted support at Alamy (a while back) about doing that and this was their reply:

"It is true that contributors cannot change the license type at their end for uploaded images.
 
But we'll be happy to change the license type of all those licensed images to RF provided you have all the relevant releases and if these images don�t have any image activity (downloads, sales) as yet. So you can get back to us with the list of Alamy refs of the images for which you would like a license type change. We will initiate this from our end and get back to you with more information."

It would seem that if a prior license term (RM) had expired you should still be OK offering as RF but I didn't ask about that.

I agree with Jo Ann.  Last year I requested Alamy to change about 20 or 30 images from RM to RF.  They did that on all but two of the images.  Those two had pending RM sales that did not show up in my stats.  They did not want to make the change on these in case the sales came to fruition.  The sales ultimately fell through and I was subsequently able to make the change.