If you want to revert your router from OpenWRT to its stock firmware, you can follow the steps provided on this page https://openwrt.org/toh/linksys/linksys_wrt1900acs. You can also do the following:
Power Switch method
This is the easiest way to revert your router to its stock firmware.
NOTE: This only applies if the OpenWRT firmware is not loaded on both firmware partitions.
Power OFF the router with the Power Switch.
Power ON the router and the Power light will start blinking. As soon as it turns OFF (for about 2 seconds), power OFF the router.
Repeat Step 2 twice.
Turn the power back ON and wait for the Power light to turn solid. It should now be booted using its alternate firmware partition.
TIP: The Power Switch method can be used multiple times, allowing you to go back and forth between the two partitions. One partition will contain the Linksys Production firmware, and the other will contain the OpenWRT firmware. You can revert from the Linksys firmware to the OpenWRT firmware (and vice versa) by using this method.
You can recover the Linksys stock firmware by using the basic SSH in PuTTy (for Windows®) and use the following commands:
Booting your router
For Secondary Partition: /usr/sbin/fw_setenv boot_part 2 && reboot
TIP: Once the Linksys firmware reverts successfully, this method will not work again as the Linksys firmware does not allow SSH port access. If you want to revert from the Linksys firmware back to the OpenWRT firmware, the Power Switch method above can be used.