In order to change the owner of a test case, click on the checkbox to left of the test case name and click Change Owner. Upon click of the clicking Change Owner link, a popup will appear that shows all of the users to whom you can assign the selected test case to. Once you select a user from the dropdown drop-down presented and confirm, that user will become the new owner of the test case. Only owner of the test case and a user with admin role can perform this operation.
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Alternatively, you can click on open the test case name and navigate to go to the edit screen for that test case and click on the Info tab.
Then, you can Information tab to change the owner of the test case by selecting another user from the owner dropdown and save.
dropdown list highlighted below:
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