In my 2006 chevy hhr i changed the oil but it still says "change oil soon".i heard that there is like a switch?where can i find that switch?

This is in the owner's manual that you should have for that vehicle. If you do a Google Search you should be able to find a free PDF version of the manual just as I did in order to get this information for you.

The Engine Oil Life System calculates when to change
your engine oil and filter based on vehicle use.

CHANGE OIL SOON MESSAGE - HOW TO RESET

After changing the engine oil, the system must be reset
as follows:
1. Turn the ignition to RUN, with the engine off.
2. Press the information and reset buttons on the DIC
at the same time to enter the personalization menu.
See DIC Vehicle Personalization on page 3-45.
3. Press the information button to scroll through the
available personalization menu modes until the
DIC display shows OIL-LIFE RESET.
4. Press and hold the reset button until the DIC display
shows ACKNOWLEDGED. This will tell you the
system has been reset.
5. Turn the key to LOCK.
If the CHANGE OIL SOON message comes back on
when you start your vehicle, the engine oil life system
has not reset. Repeat the reset procedure.

It should be in the owners manual, but I found this with a google search:

http://www.synthetic-direct.ca/technical/reset_oil_service_light_chevrolet.htm

Turn ignition to run engine off. Display OIL LIFE RESET in the Driver Information Center (DIC). Press and hold the ENTER button for at least one second. An ACKNOWLEDGED’ display message will appear for three second or until the next button is pressed. This will tell you the system has been reset. Turn the key to OFF. If the “CHANGE ENGINE OIL” message comes back on when you start your vehicle, the engine oil life system has not reset. Repeat the procedure.

It will likely be a small push button switch found in the fuse panel. What the other answers have told you is how to use it. Consult your owners manual to be sure.

resetting your oil engine is very simple, you just need proper operation of the oil life warning system. first, you need to turn on the ignition system without starting your car. then, while the oil life is displayed, try to press and hold the driver info for just one second. once the successful message is received turn the key to the off position. then restart your car to know that the change engine oil has been reset.
just pm me if you have other questions.