Kernel in use does not match headers after manual update

Question

Goal: configuring my RPi 4 to act as a bridge/router between the cafe wifi downstairs and my private network while also using Pi-hole for my local network. Effectively, my wlan0 connects to the cafe (WAN), and I have a TrendNet adapter wlan1 to broadcast my LAN. I am also trying to share wlan0 with eth0 for a NAS.

Was playing around with overclocking and one article advised running rpi-update, which I did sadly (ignoring its warnings as well as many other articles). I found out soon after that my TrendNet was not getting picked up and figured the new firmware was not compatible the old driver. No problem, I thought, as I tried to reinstall the driver but was met with Your kernel header files aren't properly installed.

I then followed steps to revert rpi-update using sudo apt-get install --reinstall raspberrypi-bootloader raspberrypi-kernel, but after reboot found that the kernel did not change. rpi-update had brought me to version 6.6.25-v8+, while the --reinstall installed 6.1.21-v8+. Fiddling around with boot config has yielded no fruit. Looking for an answer before I resort to wiping and reinstalling RPi OS Bookworm.

Main issue: uname -r always gives 6.6.25-v8+ after rpi-update but I want to roll back to 6.1.21-v8+. How is this possible?

Edit: Additional info

$ ls /usr/lib/modules
6.1.0-18-arm64       6.1.0-rpi8-rpi-2712  6.6.20+rpt-rpi-2712  6.6.25-v8-16k+
6.1.0-rpi7-rpi-2712  6.1.0-rpi8-rpi-v8    6.6.20+rpt-rpi-v8
6.1.0-rpi7-rpi-v8    6.1.21-v8+           6.6.25-v8+


$ cat /boot/firmware/config.txt
# For more options and information see
# http://rptl.io/configtxt
# Some settings may impact device functionality. See link above for details

# Uncomment some or all of these to enable the optional hardware interfaces
#dtparam=i2c_arm=on
#dtparam=i2s=on
#dtparam=spi=on

# Enable audio (loads snd_bcm2835)
dtparam=audio=on

# Additional overlays and parameters are documented
# /boot/firmware/overlays/README

# Automatically load overlays for detected cameras
camera_auto_detect=1

# Automatically load overlays for detected DSI displays
display_auto_detect=1

# Automatically load initramfs files, if found
auto_initramfs=1

# Enable DRM VC4 V3D driver
dtoverlay=vc4-kms-v3d
max_framebuffers=2

# Don't have the firmware create an initial video= setting in cmdline.txt.
# Use the kernel's default instead.
disable_fw_kms_setup=1

# Run in 64-bit mode
arm_64bit=1

# Disable compensation for displays with overscan
disable_overscan=1

# Run as fast as firmware / board allows
arm_boost=1

[cm4]
# Enable host mode on the 2711 built-in XHCI USB controller.
# This line should be removed if the legacy DWC2 controller is required
# (e.g. for USB device mode) or if USB support is not required.
otg_mode=1

[all]

# Uncomment to overclock. Default CPU is 1500, default GPU is 500
over_voltage=6
arm_freq=2100
gpu_freq=700

Answer 1

sudo apt update; sudo apt install --reinstall libraspberrypi0 libraspberrypi-{bin,dev,doc} raspberrypi-bootloader raspberrypi-kernel will put it back to the latest supported kernel/bootcode (which is currently 6.6.20 on Pi4 Bookworm and this is what you would get if you re-install).

See https://www.raspberrypi.com/documentation/computers/os.html#rpi-update which suggests sudo apt-get update sudo apt install --reinstall raspi-firmware for Bookworm.

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sudo apt list --installed | grep headers will show installed headers.
Any which say "auto-removable" can be deleted with sudo apt auto-remove