Account → Plans shows Your current plan. Change plan moves you to another one, and you can cancel from the same place.
Moving up
Takes effect straight away — more course slots and more credits from the moment you pay.
Credits you have not used come with you. Changing between two paid plans part-way through a month keeps whatever is left of that month's credits, so there is nothing to use up first.
Cancelling
You are asked to confirm; Keep Subscription backs out.
You keep everything you have already paid for until the end of that period. Nothing stops on the day you cancel.
After that your account moves to the free Fresher plan: 2 courses and 40 credits a month.
Changing your mind after cancelling
Turn it back on from the same page any time before the period ends, and nothing changes at all.
What happens to your work
Nothing is deleted. Your marks, summaries, answers and tests all stay.
If you were reading more courses than the free plan holds, they stay in your account and are there again when you move back up.
Credits when you move down
The credits your plan gave you go with the plan. Credits you bought as a top-up stay — you paid for those separately, and they run for a month from the day you bought them.