Hey, sorry for delay. Touch control may be customized by manual file edit, its may be very complicated.
you need to save file https://raw.githubusercontent.com/kasymovga/rexuiz/master/rexuiz-data.pk3/dptouc... to Android/data/com.rexuiz.app/files/data/. Edit it in any text editor, and save changed dptouchscreen.cfg to Android/data/com.rexuiz.app/files/data/
PS there is no manual about editable values.
Actually this game supposed to be played using kbm(keyboard+mouse) or at least controller both may be connected to mobile using bluetooth or usb-otg cable. Touch control very unsuitable for fast paced shooter game.