The current Service Pack for SQL Server 2008 R2 is SP3:

http://www.microsoft.com/en-us/download/details.aspx?id=44271

Steps:

1) Download SP binaries

2) Apply in test environment

3) Test applications

4) Apply in production

If it is a clustered instance, apply on the passive nodes first and the active node as last.

The Service Pack setup is just as easy as "next/next/next". You'll be asked which instances to patch, nothing more.