Hi, if the emulator is WinUAE, you can go to WinUAE's folder, then Configurations > Host. There's a couple of files there that you can edit (opening on Notepad: Fullscreen.uae), where gfx_display=0, you can alter to 1 or other values. Save, test, try values of 1 or 2 until it displays on your correct screen.
If you're on the FS-UAE one (the Win64 build), then you may also download the FS-UAE launcher that has the launcher window (the little button at the corner let you choose the screen): 