Lam Hung Chung,
You can configure an access point to have a preference, in order, for up to 3 controllers. When the AP boots it will use these configured preferences to find a controller.
If you configure the AP's preferred Primary Controller as the local (regional) controller and the preferred Secondary Controller as the headquarters then when the regional controller is not available the AP will try to register to the headquarters controller.
You can configure this on an AP-by-AP basis, but this can be cumbersome and slow. If you have WCS or NCS, then you can use AP Templates to apply these changes on a site-by-site basis (by selecting multiple APs at a time), which will save you a lot of time.
Here is a screenshot for doing this manually on each AP:

- Choose the Wireless tab on the top menu bar
- Choose All APs under Access Points in the left nav window
- Select the AP to be modified
- Choose the High Availability tab
- Enter your Primary and Secondary controller Names and Management IP Addresses
Justin