When I try to (not from terminal) the following error message opens up:
An unresolvable problem occurred while initializing the package information. Please report this bug against the 'update-manager' package and include the following error message: 'E:Malformed line 60 in source list /etc/apt/sources.list (dist parse)'
Using Ubuntu 12.04, thanks.
Edit: Went to line 60, this is what I found:
deb http://archive.canonical.com/precise partner
Edit: Commented the line out, software manager works fine now.