Use Bulk update to make the same changes to multiple test cases at the same time.
Bulk update can be done by going to Test Case page by selecting
Multiple test cases using checkbox manually, or
Clicking checkbox in Table header to select all test case
When you click the checkbox in table header then only the records loaded so far are selected. First 50 records are loaded initially by default and when you scroll the list down the next batch of 50 records load.
If you wish to select all test cases then click on "Select All" link on bulk action bar
Available Bulk Update Options
Change Test Suite Folder
Change Priority of test cases
By default add tags for all test cases; "Overwrite existing tags" can however be used if all the tags previously associated with selected test cases are to be overwritten by the ones you selected / added on this form
Duplicate test cases
Delete test cases
Note: the bulk actions are carried out by a background process so you would be shown progress of completion of task in percentage.