Evaluation edition expired
You have to install SQL Server 2008 r2 Standard edition side by side (if you want to do it on the same server) and then move the database using backup/restore method.

Below is an outline of what needs to be done (if you want to do it on the same server without installing a side by side SQL 2008R2 instance).


backup all database on Enterprise Edition &amp; script out the logins as well.
uninstall Enterprise Edition 
install Standard Edition  
Patch it to latest SP
restore databases
Create logins and map orphan users (if any).


Also, Unless you're using Enterprise-only features, you should be able to use detach and attach method from Enterprise to Standard.

Refer to answer by Jonathan here.

Also, refer to this approach with scripts... BUT WITH EXTREME CAUTION !! ... it uses detach/attach method and if you are using any enterprise features (which wont work in Standard edition) then the attach wont work.

Also, refer to Version and Edition Upgrades Matrix.