I added a number of bills as expenses using the mobile app, can I change the type to bill using the online pc app?

You cannot change an Expense to a Bill. As an Expense it presumes that the money has left your hands, that the purchase has been made, that you owe the supplier nothing more.

And you can only Pay Bills properly with a Bill Pay Cheque. You cannot reassign a regular cheque already posted.

Start by entering the Bills that are to pay in the future. Then delete the expense you created in error.