Your Time Capsule continues running its original, stable Apple software.
Before attempting any firmware modifications, spin up the internal drive and back up any legacy data. If you are using the serial method, you will need to gently peel back the rubber base of the Time Capsule, remove the proprietary screws, and extract the logic board. Step 2: Accessing the Bootloader apple time capsule custom firmware
Disclaimer: This process carries a risk of bricking your device. Proceed at your own risk. Back up all data from the internal hard drive before beginning. Step 1: Disassemble the Time Capsule Your Time Capsule continues running its original, stable
Unlike standard consumer routers, Apple did not provide a web interface to upload custom firmware images. There is no simple "Firmware Update" button to exploit. remove the proprietary screws