You should build a firmware with unlock the art partition and change it.
If you use OpenWRT-18.06 or glinet repo, you can edit the file target/linux/ar71xx/image/generic.mk, and remove ro flag for art. It would looks like:
define Device/gl-ar300m
DEVICE_TITLE := GL.iNet GL-AR300M
DEVICE_PACKAGES := kmod-usb-core kmod-usb2
BOARDNAME := GL-AR300M
IMAGE_SIZE := 16000k
MTDPARTS := spi0.0:256k(u-boot)ro,64k(u-boot-env),16000k(firmware),64k(art)
endef
TARGET_DEVICES += gl-ar300m
And compile it. Finally, you should write a shell script to change this position.
