SQL Server Agent Jobs: How to execute a job step without executing the entire job

"Start job at step" will start the job at the step you specify. However - if you don't wish to execute any subsequent steps - be sure to adjust the step logic so that it will "Quit reporting success" after completing the step you started at.

Use this:

EXEC msdb.dbo.sp_start_job N'job_name' , 'step_name'