But we can do by using below two ways:
1. Drop computed column and recreate
2. GO To Table in Object Explorer , right click on table and open in DESIGN mode.
Identify the column and edit "COMPUTED COLUMN Specification > Formula " property .
Second method will loose all existing value in computed column so best is to go for first method, it will drop, recreate and populate column again.
But never forget to note computed column expression before dropping column.