For anyone wondering, I've tried both of these methods.
NVIDIA GPU: edit "C:\Users\username\Saved Games\DCS...\Config\options.lua", set "fulscreen" to "false". Save. Enjoy.
AMD GPU: edit "C:\Users\username\Saved Games\DCS...\Config\options.lua", set "fulscreen" to "true". Save. Run the game through Radeon/Adrenalin software.