It is not currently possible. Prepaid and Current accounts will be merged at a future date as this is a testing phase. Top-up is not currently available on Current account. You can remove money from Monzo prepaid via ATM or using another app like Revolut (then to bank account).
The easiest way to think about it is it’s the prepaid app, without the features that’re mentioned here & with some bolted on functionality (not the final design), to enable you to access the current account functionality.
Targets aren’t listed as one of the features that aren’t available in the blog & they do work in the CA app
Your options for handling the balance on your prepaid card are detailed here -
Remember that the preview is a test & it’s not supposed to be the finished product. So that’s probably not the only piece of functionality that hasn’t been set up the way you’d like it to be or in the way it will work, for version 2 of the app.