How to change iphone maps to Google maps? (iPhone maps sucks!)

The iPhone maps programs *****! I have to use a GPS programs to find my customers. The iPhone maps programs directed me to the middle of the Everglades more than once when that is not where I was going. Most of the time it just gets the address wrong and takes me to the back side of the house/business or in most cases it takes me miles aways from where I need to be.

How can I change the default map function to Google maps?