Upgrading SQL Server 2008R2 to SQL Server 2012 in 5 minutes

Link to downloadables i.e. SSMS 2012 and Express databases
https://www.microsoft.com/en-us/download/details.aspx?id=29062

Official list if potential issues during an upgrade
http://technet.microsoft.com/en-us/library/ms143179(v=SQL.110).aspx
http://msdn.microsoft.com/en-us/library/ms143179.aspx

You may also want to check the discontinued features page:
http://technet.microsoft.com/en-us/library/ms144262

Run the Upgrade Advisor before you upgrade
http://technet.microsoft.com/en-us/library/ms144256.aspx