This is a costly upgrade where you trade in your old 60 kWh battery for a new 85 kWh battery. Tesla would value your current battery based on mileage and time.
In one case, with a 10-month old battery with 11,000 miles, the total cost of the upgrade was $18,400.
As of April-2014, Tesla no longer offers this upgrade, but you can always ask. Tesla does change its mind on occasion!
There are also third parties that take a salvaged car battery and will replace your smaller battery. eBay is one place to search for what’s available.