The question.

Which variant will be correct?

DID YOU IMPROVE THIS UNIT?
OR
HAVE YOU IMPROVED THIS UNIT?

Past simple or present perfect?

A lot of thanks.

<em>BOTH ARE CORRECT. THERE IS A VERY SMALL DIFFERENCE IN EACH. </em>

"Did you improve this unit", would be an example when someone has finished the unit and they are being asked, "Did you improve this unit?" and the response would be "Yes I did."

"Have you improved this unit", would be an example when someone is still studying the unit, and they are asked, "have you improved this unit". And the response would be, "Yes I have." (and I am still learning).