I want to download an history of my all tasks completed.
is there any option to do that.
Unfortunately not, at least not directly, but you could install a "print to PDF" software, access the "View All Completed" list (through the Karma score drop-down), use "Print current page" option from the gear icon menu and choose to print to a PDF document.
If you can, I would find it helpful to be able to download as a CSV or excel so I can analyze my data the way I want.
+1 to CSV or XML export
+1 here as well.