depending on what u wanna do.
if you wanna drop it altogether then yes "drop".
if you wanna swap it for a different subject, OR change your lecture/tute times for the same subject, then make sure you pick "swap", and then enter in the course code, whether it's for a different subject or for the same subject.
this is so that in the 1 min it takes for you to "drop" maths (for example) and then "select" maths again, the spot you wanted to get might hvae been taken, so then you'd be like 'ok i spose the other one wasnt too bad', but you try to enrol in that and THAT ONE'S TAKEN TOO! (it happens)