There is a subscription period for every IPO. You can make the changes or cancel your IPO applciation within this period. Post this; there is nothing that can be done.
There is a slight chance of you revising or cancelling your IPO application provided that the IPO has still kept their subscription open. IPO isn't like a regular stock trading where you can simply sell and buy things as your wish. In an IPO, there are steps and procedures to be followed, which have a definitive meaning.
In an IPO, if you ever find yourself confused about whether you want to pull back from the IPO or merely cancel your application, then you can do so. You can fill in the revision form stating your interest in the change of cancelling action that you want to persevere for your IPO application. However do keep in mind that if the subscription or the change period is over, then you cannot do anything about the application.
Once you fill the form, you will have to submit it to the syndicate member to make all the changes you have entered in the application. Don't worry, all of this will not incur any changes; but then based on the IPO, the number of times this change is possible can be limited.
Making a change or cancelling the submitted IPO application depends on the IPO you have applied for. Typically many have a subscription period before which all necessary adjustments can be made. But after that, you cannot make any changes nor anything can be done about it.