A test case may require change in its data and an authorized user can make these changes.
Open the test case which you want to edit.
Click Edit icon.
Changes can be made in values for all the fields, since editing and adding test cases share common procedures, the article on adding test case may be referred for further details.
Test cases being the most important resource need a better tracking of changes they have been through, so every change in a test case results in a new revision. With revisions you can have the details of changes a test case had been through since it was added to the project.
What if a test case added to a test plan gets edited?
The tester running that test case would be notified about the availability of its newer version, and he would have option to either use the older version of test case he/she is being shown or he can update to the latest version if he likes to.