Try following trick...

update emp set empid = 100 where empid = 10

The above statement gave me following error as empid is identity column

Cannot update identity column

Hence I did following and achieved the effect of update

set identity_insert emp on

insert into emp (empid,empname,designation) select 100,empname,designation from emp where empid = 10

delete emp where empid = 10

set identity_insert emp off

* Important Note : If your identity column is also a primary key and is being referenced as foreign key with CASCADE UPDATE and CASCADE DELETE enabled in detail table, please first drop the constraint or disable the CASCADEs before running the above trick else at Delete statement above it will delete records with empid = 10 in detail table which you won't expect for Update.