Index rebuild after SQL Server upgrade to 2014
You can just rebuild indexes as you normally would as part of your standard maintenance schedule, this doesn't need to be done specifically due to the upgrade. That being said it depending the size of your tables\indexes and how much time you have for your upgrade, it can't hurt to start with fresh rebuilt indexes if you can work it into your upgrade plan. But no, it's not a requirement.

You definitely need to update your statistics however, as this can lead to poor plan generation by the optimizer after upgrading. This needs to happen before you do any index rebuilding. I'll try and dig up the link I had that gave a more detailed explanation.

Here is a great article by Thomas LaRock that can serve as a basic upgrade checklist to build on, to ensure you haven't missed anything: 

http://thomaslarock.com/2014/06/upgrading-to-sql-server-2014-a-dozen-things-to-check/