pi64-update installs kernel predating 3b+ about pi64

[Crazyhead90]

You could try following this guide:
https://devsidestory.com/build-a-64-bit-kernel-for-your-raspberry-pi-3/

Thats what i used to manually update the 3b to a 3b+ version

Instead of downloading the image in the guide, download my image and follow the kernel 4.8 part in the guide, but download 4.16 instead

[satmandu]

It looked like 4.14 was getting more recent updates so I ended up doing something like this to update firmware & kernel:

sudo apt update 
sudo apt-get install -y bc build-essential gcc-aarch64-linux-gnu git unzip subversion
mkdir rpi ; cd rpi
# to get head, aka most actively developed current rpi kernel tree
git clone --depth=1  https://github.com/raspberrypi/linux.git
# for specific version use: git clone --depth=1 -b rpi-4.14.y https://github.com/raspberrypi/linux.git
# Now we get updated rpi firmware
svn export https://github.com/raspberrypi/firmware/trunk/boot
rm boot/config.txt
rm boot/kernel*
cd linux
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcmrpi3_defconfig
ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j`nproc` deb-pkg
cp arch/arm64/boot/Image ../boot/kernel8.img
cp arch/arm64/boot/dts/broadcom/*.dtb ../boot/
cd ..
echo "kernel=kernel8.img" >> boot/config.txt

That should give a bunch of deb packages which can be installed on the target system as well as a boot folder which can be used to overwrite the boot folder on the target system. (This works for both 4.14 and 4.16, but I only tested 4.16 via ssh console so can't vouch for functionality like audio or video.)