Hi there!
Tools like Web2CRM and PowerWebForm will bring the data into CRM and create a new Account. But updating an existing Account is the hard part. The idea of creating a portal is mentioned above because in that way, people can login and then you know the exact Account they associated with. However, that creates extra steps for the user.
Alternatively, you would have to create some rules that match an incoming application with an existing Account. This would be custom code or using a tool like Scribe to intercept the entry and determine whether the Account is existing or new.