Update SP - check if value exists first

ALTER PROCEDURE [dbo].[sp_Trip_UpdateIgnitionOff] 
AS
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;

    -- Insert statements for procedure here
	

UPDATE tm
SET nEndEventInst = td_outer.vSequenceId, dEndTime = td_outer.dGPSDateTime
FROM tblTrip_Master tm
CROSS APPLY (
    SELECT TOP (1) td.vSequenceId, td.dGPSDateTime
    FROM tblCommonTrackingData td
    WHERE
        td.vReportId = 8 and
        --td.vSequenceId > tm.nStartEventInst AND
		td.dGPSDateTime >= tm.dStartTime and
		td.vpkDeviceID = tm.vpkDeviceID and 
		td.dGPSDateTime < DATEADD(SECOND, -30, SYSDATETIMEOFFSET())

    --ORDER BY td.vSequenceId
	ORDER BY td.dGPSDateTime, td.vSequenceId
) AS td_outer
WHERE
    tm.nEndEventInst IS NULL
	

END

WHERE
    tm.nEndEventInst IS NULL

--here is the bit i think needs changing
AND 
--do any rows with tblTrip_Master.nEndEventInst  already exist (also need to pass in [vpkDeviceID] with this)
(if exists: Select count(*) from tblTrip_Master where tblTrip_Master.vpkDeviceID  = td.vpkDeviceID AND  tblTrip_Master.nEndEventInst = tm.nEndEventInst
If COUNT  = 0 then proceed with update
Else - exit without updating