By law, the assessor's value on your property is presumed to be correct. that presumption can be overcome only by the presentation of clear, cogent, and convincing evidence, which is evidence showing that it is highly probable that the assessor made a mistake. To prove your case, present sales of properties that are similar to yours. The sales should be as close to the assessment date as possible.
Understanding and appealing your property assessment can be confusing. Below is a link that will help you understand the process a little more.
Appealing Your Property Assessment - Publication - Helpful Information