Hi @jimin han ,

To reset any form layout I would simply goto "Configure > Form Layout" Then click on "Show Versions" under Related Links. This will show you a list of all your changes and you can pick any change record   based on timestamp and then choose "Revert to this version".

Or, in other words:

When you update a form layout using the form design, it creates a Form Layout update.

Versions of this form layout are stored in sys_update_version (you can go in this table by typing sys_update_version.list in the filter navigator).

Inside this list, order by Created Date/Time to see the latest updates and filter with a Name that starts with sys_ui_section.

Once you find the right record (See the field Record name which will contain the table name (Contract)), go in the Version related list and revert to the Previous version which was OK.

Mark my answer correct & Helpful, if Applicable.

Thanks,

Sandeep