How to Update multiple columns with using ABS,DATEDIFF

Question

DECLARE 

@deviationtime int,
@actualtime datetime,
@estimatedtime datetime


While (Select TOp 1 successrate FROM YBS_Tahmin_Kayit where deviationtime = 999 order  by recordtime desc) = 999
    BEGIN 

    SET @actualtime= (SELECT Top 1 actualtime From [TTS].[dbo].YBS_Tahmin_Kayit where deviationtime = 999  order by recordtime Desc  )
    SET @estimatedtime =        (SELECT Top 1 estimatedtime From [TTS].[dbo].YBS_Tahmin_Kayit where deviationtime = 999 order by recordtime Desc  )



        SET @deviationtime = ABS(DATEDIFF(second, @actualtime, @estimatedtime))



    Update [TTS].[dbo].YBS_Tahmin_Kayit SET deviationtime = @deviationtime where deviationtime = 999 and estimatedtime= @estimatedtime

    end

I SETTED ALL deviationtime = 999 in database.

I want update all deviation time from database but my code making 1 by 1.