How do I go back to mainstream package?

Hello, you can use 'dpkg -r mypackagename' and then do a usual apt-get install ....

It isn't as easy that for me, as it has dependencies, the package is gnome-system-tools and the dependences are ubuntu-desktop and fast-user-switch-applet. I would like to replace it with the current version. The current version is not in the main part of synaptic but in the section of "local or obsolete".

Then download the .deb you wish to install and use 'dpkg --force-all myfile.deb'

or try 'apt-get --reinstall install mypackage'

I get this output when I try to install it:
michael@michael-laptop:~$ sudo dpkg --force-all /var/cache/apt/archives/gnome-system-tools_2.22.0-0ubuntu9_i386.deb
dpkg: need an action option

Type dpkg --help for help about installing and deinstalling packages [*];
Use `dselect' or `aptitude' for user-friendly package management;
Type dpkg -Dhelp for a list of dpkg debug flag values;
Type dpkg --force-help for a list of forcing options;
Type dpkg-deb --help for help about manipulating *.deb files;
Type dpkg --license for copyright license and lack of warranty (GNU GPL) [*].

Options marked [*] produce a lot of output - pipe it through `less' or `more' !

'dpkg --force-all -i myfile.deb'

or

'apt-get --reinstall install mypackage'

Thanks Christophe Painchaud, that solved my question.