On the first day of your next cycle. 

You could remove it earlier. Just depends on when you need it until

If the data add-on is part of shock free data then it should be dropped next billing period. If the data add-on is added by other means then you will need to remove it from the add-ons page in your self serve 

How and when I can change my monthly plan to no data usage?

i simply want phone calls and receiving.

Are you a prepaid or postpaid customer? Have you tried to change your plan in selfserve yet?

No. As a matter of fact, there are such options under Self Serve of my Koodo.

Do you get error message?

Did you try logg-in > self serve > mobile service > chenge your plan?

Can you post a screenshot what it looks like?