SQL Server Instance Version
Basically, you have to install SP1 and then CU6 (as mentioned in my comment)


10.50.2811 ==> 2679367 Cumulative update package 6 (CU6) for SQL Server 2008 R2 Service Pack 1 
10.50.1765 ==> 2489376 Cumulative update package 6 (CU6) for SQL Server 2008 R2 note this is for RTM