To Download SQL Server 2012 Express:
Please refer Link:
https://www.microsoft.com/en-my/download/details.aspx?id=43351
Click Download.
Choose x64 if your Server PC is 64bit or choose x86 if your Server PC is 32bit > Click Next.
To Upgrade from existing SQL Server 2008 R2 Express to SQL SERVER 2012 Express
Step 1 - Click 'Upgrade from SQL Server 2005, SQL Server 2008 or SQL Server 2008 R2'
Step 2 - Click Next.
Step 3 - Tick Both Option > Click Next.
Step 4 - Select Instance > Click Next
Step 5 - Click Next.
Step 6 - Upgrade Start
Thanks,