As above, no need to edit the start date - responsibility access doesn't date-track so it's just used as a facility to future date access requests. Removing the end date (or altering it to whatever date in the future you want the access to cease) will suffice.
when i click on end date,there is no option to remove the end date(there is already end date:1-jan-2017), how to remove the end date?
You cannot add same responsibility again to same user. You need to either remove end date (it should work, I tested) or delete the row and add again (if system allows you to delete).