how to change my plan to pay as you go?

You need to go to my plans, then manage, then adjust my plan, there you have to remove your voice, data, text plans. It will take effect next cycle. At that point, you will put money in your wallet and use it as you go

Please note that you must minimum have a plan with data if you want to be able to receive data.
To change your plan:
Go online and open your account:
Take only data, erase unlimited minutes and textos. And click: save.
Or, if you really want nothing specific in your plan, erase every thing, (data, textos and minutes). The minimum plan will cost you monthly 10.46$ + tx.
And if you need data, minutes or textos , you will add money in your wallet in advance and choose add-on by blocks of what you want (minutes, textos or data).