Sorry if I'm not following you correctly here - are you saying that the lead can't change the value in the form? That doesn't make sense to me, the lead should just be able to change the value in the form before hitting submit.
Bear in mind tho, that you can't update the Billing Details of an account from a form fill out - Marketo doesn't write to the account record (Read-only fields in Marketo). You can ask for this to be enabled but it's not generally advised.
If your updating address details on the contact (e.g. Mailing Address) and not seeing the fields update, check you haven't blocked field updates from form fill outs in Admin>Field Management.
Hope this helps.
Thanks for your answers. The main issue is that I can't show the values coming from Marketo prefilled in a Form on a Landing Page. The showed prefilled information is coming from Google Chrome information.
Its my intention to show in a Form the actual information coming from Marketo and that the customer only has to change the fields whats wrong in the same Form....
Hope you can help.
PS when I submitted for a Marketo Roadshow when i filled in Honeywell... Marketo itselfs came up with some suggestions of Honeywell locations so there must be a way to prefill a Form.... but the questions is how....
Perhaps the most easy solution for you problem is setting the value of the postal code to NULL , basically delete it. If you then make the field required on the form they would have to resubmit it.
Or option B would be to write incorrect data value into a secondary field, for example Incorrect_Postal code, and then blank out the original field.
This would still allow you to display the incorrect value on the form and have new input area for the correct box below.
Maybe I will use your second options but that looks not very professional....
My understanding is that you set pre-fill on the form itself (Forms 2.0) but that it only works on a Marketo landing page. The pre-fill options that pop up with possible addresses is a different app I think, possibly ReachForce.