sql server - How to modify my T-SQL UPDATE query by adding a CONDITIONAL clause? -

i'm using sql server 2014 , have update query runs on table called "resstaydate" (extract shown below):

id      staydate        rateamount ----------------------------------- 256     2015-11-28      248.00 256     2015-11-29      248.00 256     2015-11-30      248.00 256     2015-12-01        0.00 256     2015-12-02        0.00 256     2015-12-03        0.00 

i need update rateamount column , update query follows:

use mydatabase  update resstaydate set rateamount = case resid                     when 256 155.00                     else rateamount                  end 

my problem if run query "as is", update rows 155.00. need update rows staydate in december 2015.

output should this:

     id      staydate      rateamount     ---------------------------------     256     2015-11-28      248.00     256     2015-11-29      248.00     256     2015-11-30      248.00     256     2015-12-01      155.00     256     2015-12-02      155.00     256     2015-12-03      155.00 

how modify query updates part of resstaydate table?

if want affect records in december, use datepart function:

use mydatabase  update resstaydate  set rateamount = case when resid = 256 , datepart(month,staydate) = 12 155.00  else rateamount  end