How can i update my asus router rt-ac58u to OpenWrt

Need help for i cannot configure my asus routert rt-ac58u to my desired ovpn file. I have read flashing the unit but need serial cable to do it. Anyone who can have it through firmware? Your help will be greatly appreciated. Thanks admin

Installation instructions:

There is already a bunch of topics regarding RT-AC58U installation. Please use the forum search.