Can I download the student's work, grade it and upload it again?
You can download all of the student submissions for file uploads, text entries, website URLs , and Google Doc submissions. You can then comment on them, and re-upload them into Canvas.
Here are two lessons from the Canvas Instructor Guide that explain how to do this.
This is not done through SpeedGrader, which acts on a single student at a time.
Assigning a grade is a separate process. If you want to bulk upload grades, the best way is to export the gradebook, open it in Excel or another spreadsheet, adds the grades, and then import the grades back into Canvas.
Here are some lessons from the Canvas Instructor Guide that explain how to do this.
Retrieving data ...