Hi Kanwal,
Update activity,

Highlighted area is used for reconciliation for dynamic values if you want to update values for a particular type of records you can use the other activity in which you can use drop down.
Use split or other activities to apply a filter which will give you this flexibility to use drop down like below screen shot.

you can also rely on distribution values in some scenarios like below:

Some time distribution values are beneficial in workflows to select correct options from the data base.
All you have to do is refine your workflow a bit.
Regards,
Amit