Hi Sebd.DD,
When you say overwrite, do you mean undo the managed components, with an unmanaged solution? If so, I'm afraid you cannot do this. The nature of managed solutions is such that they protect the contents of the solution from being tampered with.
I would suggest deleting the Managed solution, and then importing your Unmanaged solution, if you'd like it to be Unmanaged moving forward.
Please take a default solution back-up of your Target system, so that if anything goes wrong, you can revert.
Thanks!