>>SQL Server is just a servce. You can stop it from Service Manager. or from SQL Server Configuration Manager.
>>If SQL Server Configuration Manager is not in your Start menu go to Windows\System32 forlder and find it.
>
>First, thank you. How do I find which service to stop? you see, I have both SQL Server 2008 running on this PC and SQL Server 2014 Express running on this PC.
>When I open the SQL Server Configuration Manager, I see SQLEXPRESS and MSSQLSERVER running. However, when I log into the SSMS, I see DMITRY2013 (when I try to use SQL 2008) and I see DMITRY\SQL_2014_EXPRESS when I am using SQL 2014. I want to be sure I stop the right service.

How do you connect to SQL Server?
What is your connection string?
You use only Server=DMITRY2013
or
Server=DMITRY2013\SQL_2014_EXPRESS