I am developing a software using Quickly. I have changed the icon but unity is still showing the old default icon even after I log out and log in back. I have installed a .deb file of the software and it contains the new icon. I have even checked the .desktop file, which points to the new icon.
How to change the icon from Unity's memory?
I, however, am not sure whether this is Unity only.
Usually, when the Icon= line is changed in a .desktop file, it should be visible in the launcher after log out/log in. When an application is re- installed its icon should change as well, there is no "Unity memory" as you mention it.
Another thing is that a local version of a .desktop file (in ~/.local/share/applications) has priority over the global one in /usr/share/applications. Since you mentioned having an (old) local copy of your .desktop file (in a comment), your newly defined icon in the global .desktop file only takes effect after you removed the local .desktop file.
~/.local/share/applications? (old version)