You will need to connect to the device via a console cable such as this: https://www.amazon.com/CAB-CONSOLE-RJ45-Cisco-RJ45-DB9/dp/B00SWPR4VI/ref=sr_1_5?dchild=1&keywords=cisco+console+cable&qid=1612929042&sr=8-5
If you don't have a serial port on your computer, you'll need a USB to serial adapter such as this one: https://www.amazon.com/HDE-Interface-Console-Adapter-Routers/dp/B00HUZ6OMQ
After you connect to the console port, you'll also need a terminal emulator such as PuTTy found here: https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
Now that you are able to access the command line interface, you'll need to find out what version of code you are running as to determine whether you can program this access point (autonomous image) or if it has to be managed by a wireless LAN controller (Lightweight Image).
ap3g2-k9w7-mx.153-3.JPI4 = Autonomous Image
ap3g2-k9w8-mx.153-3.JPI4 = Lightweight Image
Please let us know what version is running on your access point by issuing the command
AP# show version
