Because there can coexist alternative versions of the same program on a Debian system (especially programs like qmake and java), there are symbolic links from /usr/bin to the desired versions of these programs.

To manage these links, and change them to point to alternative versions, root can use the command update-alternatives.

For example, to choose a default version of qmake to run, root can use

update-alternatives --config qmake

To see a list of alternatives for each installed program that has alternatives, root can type

update-alternatives --all

and select defaults for all of the alternatives.