If you have questions about your assessed value, the first step is to call the Madera County Assessor's Office at (559) 675-7710 and ask to speak to your appraiser. You can discuss with the appraiser why you feel the value is incorrect and possibly come to an agreement.
If you just purchased or made changes to your property, you can apply to appeal your assessment, you can have your appeal heard by the Madera County Assessment Appeals Board. The application is available on their webpage. Be sure to review the filing dates on their webpage to be sure you are still able to file an appeal.
If you feel that your property is assessed higher than the current market value, you can apply for a temporary reduction in value (this link is to a form that can be filled out and printed). Your property would be lowered to the current market value, and reviewed annually until it regains the Factored Base Year Value it would have without the temporary reduction. Be aware that the reduced values are not limited to the 2% maximum your Factored Base Year Value.