How minus one date from another date

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SQL

Hello
my question is how to change date from another date.
e.g my date is '2016-05-23 10:53:07.913' and i want date 2016-05-22 10:53:07.913(one day minus)
OR If i want change value from 39;2016-05-01 to 2016-04-30(here one day minus but also change month)
in simple word i want change one date automatically from another date,
in my code only change day.if i change date on first day it change last month day..

please share..

What I have tried:

set @StartDate='2016-05-13 10:53:07.913'
       
 set @b='2016-05-23 10:53:07.913'               
 if(@StartDate<=@b)  
    begin  
    set @abc=(SELECT DATEADD(day,-1,'2016-05-23 10:53:07.913' )) 
    (SELECT DATEADD(Month,-1,'2016-05-23 10:53:07.913' ))
         
   end  
   else  
   begin   
    set @b=@b  
   end  
   
   declare @q int=( select DATEDIFF(dd,'2016-05-13 10:53:07.913','2016-05-23 10:53:07.913'))
   select @q