guess there's no such feature on imgflip
you may have to upload your old pic while creating it again.
If you don't have it anymore or don't want to search it there's an easy short-cut: available
right-click on your old meme
then check 'copy image location' (by a left-click)
then go to the meme creator, select 'upload your own image' --> 'paste image url here' via (ctrl-v)
then it uses the old meme as your new one- you don't even have to caption it anymore.
