Answered question
I followed the guide to change my plan but when i click on manage my plan it says to click on ajust plan but there’s no option or button to do it
Might not be the case, but make sure you don't have any pop-up blocker activated.
Other answers
You will have to wait until next month to change your plan
You can change your plan anytime you want but it will only take effect on the next billing period.
Also, you won't be able to go back to your previous introduction plan after a plan change. So I suggest to use the add-on feature if you have a punctual need for more data or call minutes.
You will keep your introduction price as long as you don't change it!