Try doing it in terminal.
sudo apt-get update && sudo apt-get upgrade
Sheesh or it could also be.
sudo apt-get update && sudo apt-get dist-upgrade
Would probably try the first one, ... first. Nope it's sudo apt-get update && sudo apt-get dist-upgrade and am sure this time.
See this thread for some other poss useful lmde related stuff. Had to search to make sure which, was which. This thread just looks like a useful reference for folks running lmde in general. Was good enough that one of the mods stickied it anyway.viewtopic.php?f=141&t=76010&p=445621&hilit=hack+lmde#p445621