Hi @tenacean,
In Canvas, you can always alter the available from, available until, and due dates. The available until date controls how long students have access to continue to make submission attempts for the assignment (you can even leave it blank to allow submissions attempts until the course ends). The due date will affect what the students see on their to-do list and any late policies you have set up in the gradebook.
I'm not sure if the above will be useful or not. From your wording, I'm wondering if you perhaps gave a text entry assignment and want students to be able to go back and modify a previous submission. If that is the case, for assignments the students would have to copy.paste their old text submission into a new one. For file upload assignments, they could continue to work off of the file on their computer, modifying it and then uploading a new copy when they are ready.
I hope this helps. Let us know if this response misses the mark on what you're trying to do though!
-Chris
