If you are using a prepaid plan, it`s easier. Generally, in 30 days, since you expressed your wish to change your plan, by sending a message to the interactive menu of the provider, your option is implemented
In the case of a postpaid plan, if you have purchased also a phone, you will have to announce the provider about your intention. Usually you can choose a cheaper plan than the previous and you can`t give up to the existing for less than 12 months
