By admin do you mean the person who created the segment? In most cases
you don't have to be an admin to create a segment (permissions vary per
account). Now if you want to modify a segment that was shared with you,
you can't do that. You either 'Save As' the segment and modify it, which
will create a new segment or ask the original owner to modify it. If the
original owner is no longer an active account you can transfer the
segment to your account, but an Admin will have to do that.