Maybe it will be useful for someone doing it first time ever like me.
I will comment main instruction at https://openwrt.org/toh/aruba/ap-303
I have done it with use of old windows 7 laptop and handmade UART adapter.
Step 0:
I have used chip UART adapter PL2303HX (~ 2 eur), driver need to be v1.5. Newer driver versions not working as it some сhinese copy of original taiwainese chip...
Cutted old Micro-usb with 4 wires and just soldered it strait to UART adapater.
Step 1:
Downloaded and renamed image "ipq40xx.ari". Downloaded tftp server "TFTPD64 exe" and set-it up as it shown here https://youtu.be/YdVEonLMWYs, select folder with ipq40xx.ari. IP address for tftp is 192.168.1.75. Also changed LAN settings to IP 192.168.1.75, mask 255.255.255.0, gateway 192.168.1.105. Select 192.168.1.75 in TFTPD64.
Connect lan cable between laptop and ap303 and UART adapter.
Step 2:
Downloaded "puTYY exe".
Set it up: Category > Session > Connection type > Serial > Serial line > COM1 (you need to choose your com port number); then Category > Serial > Flow control > none; then Click on "open" button. Your will have puTYY black console opened.
Now power on ap303 with 12v adapter and puTYY console will start showing some text 
. Press enter when it start "countdown".
Step 3:
Copy lines from step 3 (without $ )
setenv bootargs_openwrt "setenv bootargs console=ttyMSM1,9600n8"
setenv nandboot_openwrt "run bootargs_openwrt; ubi part aos1; ubi read 0x85000000 kernel; set fdt_high 0x87000000; bootm 0x85000000"
setenv ramboot_openwrt "run bootargs_openwrt; setenv ipaddr 192.168.1.105; setenv serverip 192.168.1.75; netget; set fdt_high 0x87000000; bootm"
setenv bootcmd "run nandboot_openwrt"
saveenv
enter them in puTYY with mouse right click one by one.
Step 4:
run ramboot_openwrt
I have disabled laptop wifi on this step as it was struggling to start processing it.
Step 5:
"transfer the OpenWrt sysupgrade image to the /tmp folder on the device."
Was most complected for me. I have tried via windows cmd & pscp.exe without successes.. Something like this: "C:\folderwithupdate>pscp -P 22 openwrt-sysupgrade.bin root@192.168.1.1:/root/"
Finally downloaded WinSCP exe, entered in 192.168.1.1, found couple /tmp folders there and dropped "openwrt-sysupgrade.bin" file.
Step 6:
As per instruction, no comments
vi /etc/config/network
service network reload
Then changed lan settings gateway from 192.168.1.105 to 192.168.1.1.
Entered openwrt interface in browser 192.168.1.1.
p.s.
My sorry to professionals this comments aimed to dummies like me
In main instruction was noted that chip UART failed to work however my chip worked well.
