If you have a SketchUp Pro 2015 license and an active maintenance & support plan, then you should have received an email with your new SketchUp Pro 2016 license information. If not, you can request to have the license emailed to you again, or contact your local reseller if that’s where you purchased the license.
To use your SketchUp Pro 2016 license, simply download and install SketchUp Pro 2016 to your computer. Then open SketchUp Pro 2016, click the Add License button, and copy & paste your new license information. The serial number will begin with the letter “R.”