How do I edit report issue page field in mantis

I'm a beginner in mantis, please help me to solve

I want to edit some field in report issue page

1. set default value in category column while reporter report issue
2. set platform/OS/OS version in combo box and set default value, like Windows/Linux in OS

report issue page-before

report issue page - after

please help

1. Login as Administrator
2. Select "Manage" from the main menu.
3. Select "Manage Custom Fields" from the management menu.
4. In case of edit, click on the name of an existing custom field to edit its information.
5. In case of adding a new one, enter the name of the new custom field then click "New Custom Field".

Thanks :)