which the best skill to lower the contrast and saturation of a jpg cs6?

I would start by looking at Levels and Curves adjustment layers.

If you do a Google search for levels and curves tutorials you will get plenty.

Here is one to get you started. http://www.youtube.com/watch?v=XRo_5kC3GNE

Fact is that the best results are usually a combination of techniques (including the saturation and brightness / contrast adjustments).

BTW - in the main make sure you are using adjustment layers instead of adjusting the whole image as this is non-destructive.

Good luck

Cheers

John

What version of Photoshop? (it makes a difference to what tools are available)

Do they look equally over saturated when viewed outside of Photoshop?  It might be a colour space problem, for instance.  A good rule of thumb is to upload one to a web sharing site, or even to this thread.  What see on your web browser is going to be similar to how other people will see your images, screen calibration issues not withstanding.

Personally I'd switch them to 16 bits/channel mode, then use Image - Adjust followed by several different tools, e.g., Hue/Saturation and Curves.

There are those who might prefer adjustment layers, for their ability to tweak a bit more later, and that's fine too.  There are Hue/Saturation and Curves adjustment layers.

-Noel

i would first make sure you are applying the correct source profile

try Edit>AssignProfile: sRGB, AdobeRGB, ProphotoRGB, AppleRGB, MonitorRGB (five different profiles to try)

if any of those improve the color (and your monitor profile is good)...then Assign that profile and then, Edit>ConvertToProfile (probably sRGB is the safest...then Save As EMBEDD the ICC Profile)

then go to work on the color as recommended (check out Adjustment Layers)...

Among the gazillion ways to compensate for hypersaturation and excessive contrast, do not overlook a Vibrance adjustment layer.

"Gazillion" is a slang expression that generally translates to "many, many".

I more often say "bizillion" or "eleventy seven" myself. 

-Noel