A newly purchased plan won't start until the current plan has run its full monthly course and expires.
You can't start a new plan early.
You'll just have to wait until your current plan expires for the new one to start.
If you have used up your mobile data allowance on the current plan and need to continue using mobile data then you'll need to buy a data add-on.
If you need to contact 48 support about this then see the links in my signature below.
