@Add_20354 HI there & welcome to the 3Community. To change your plan you'll need to first log into your My3 account for the steps required check out our Help & Support article Change Plan or Add Ons. Once you've changed your plan you can then top-up by €30 to activate the new allowances.
Since you've only topped up recently I would suggest waiting to change plan until your current 28-days are up so you still get the full benefit of your current allowances.