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Hi fallou
Can you give us a little more details? After you removed the older MikTeX and installed the new one, did you update your editor to point to the new version?

Hi
can you give a little more detail on what you did after you downloaded the cls file and what you are hoping to accomplish? Do you just want the cls file to be available for this project? Or are you going to be using the cls file for oter projects in the future?

If you're not going to install it ...

Hi Gregory,
Can you say what exactly you did or what set of instructions you followed?

What you are supposed to do, is move the files to the correct place in your local TeX directory tree. You may have seen this referred to as the texmf-local directory. So, for Windows using MikTeX, this might be C ...

Hi
LaTeX has a Vancouver citation style which you can use and can be found here. As far as doing it in LyX, it is possible. This PDF should explain things.

Hope that helps.