Should you choose to register for the LSAT and then subsequently decide to cancel (withdraw), the LSAC has two different withdrawal deadlines.
The primary withdrawal deadline (by which you would still be able to obtain a partial registration refund) is approximately THREE WEEKS prior to the test administration. Check lsac.org for specific dates.
You also have the option to withdraw your registration for a particular LSAT administration up until the midnight prior to Test Day. This will additionally allow you to forgo taking the exam and not risk having a score cancellation or no show appear on your LSAT score report sent to the law schools to which you will apply. (Note: no refund is given for such withdrawals.) To do this, request a withdrawal through your LSAC online account.
The LSAC limits you to three “exposures” to the exam over any two year period. As a withdrawal does not expose you to the content of the test, it does not count against your 3-in-2 years allocation.