How do I use update query with case?

 update A_SalaryConsolidated set bonus=case when a.final_payable >bonus then bonus else (bonus+a.final_payable) 
 from  
  (select empid,final_payable from Bonus
  inner join monthdigit on month=monthView     where monthnumber= 5 and year = 2017
 and Bonus.empid in(select  a.empid from(select empid from Bonus 
 inner join monthdigit on month=monthView where monthnumber=5 and year=2017  and final_payable>0  and unitid=145) as a )) as a 
inner join A_SalaryConsolidated on a.empid=A_SalaryConsolidated.staffid   where month ='July' and year = '2017'