Upgrade ancient build to current Buster or Bullseye

My trusty 4B has been working in production, but it has been way too long operating on this original set of repositories (principally the radxa one). Can someone walk me through the process of upgrading the kernal and repositories to something supported and (hopefully) mainline? All the tools, etc. are ancient and of course security updates are pretty much stale at this point. I’d like to clean up the cache policy as well while I’m at it… Thanks!

~$ cat /etc/os-release
PRETTY_NAME=“Debian GNU/Linux 9 (stretch)”
NAME=“Debian GNU/Linux”
VERSION_ID=“9”
VERSION=“9 (stretch)”
VERSION_CODENAME=stretch
ID=debian
HOME_URL=“https://www.debian.org/
SUPPORT_URL=“https://www.debian.org/support
BUG_REPORT_URL=“https://bugs.debian.org/

~$ uname -a
Linux wx 4.4.154-82-rockchip-00022-gb99b90e #2 SMP Thu Apr 11 16:20:44 CST 2019 aarch64 GNU/Linux

~$ apt-cache policy
Package files:
100 /var/lib/dpkg/status
release a=now
500 http://[local]/apt squeeze/main all Packages
release o=python2 squeeze,a=squeeze,n=squeeze,l=python2 squeeze,c=main,b=all
origin [local]
500 http://repo.linaro.org/ubuntu/linaro-overlay stretch/main armhf Packages
release v=9.0,o=linaro-overlay,n=stretch,l=Linaro Overlay (stretch),c=main,b=armhf
origin repo.linaro.org
500 http://apt.radxa.com/stretch stretch/main armhf Packages
release o=apt.radxa.com,n=stretch,l=apt.radxa.com,c=main,b=armhf
origin apt.radxa.com
500 http://security.debian.org stretch/updates/non-free armhf Packages
release v=9,o=Debian,a=oldoldstable,n=stretch,l=Debian-Security,c=non-free,b=armhf
origin security.debian.org
500 http://security.debian.org stretch/updates/contrib armhf Packages
release v=9,o=Debian,a=oldoldstable,n=stretch,l=Debian-Security,c=contrib,b=armhf
origin security.debian.org
500 http://security.debian.org stretch/updates/main armhf Packages
release v=9,o=Debian,a=oldoldstable,n=stretch,l=Debian-Security,c=main,b=armhf
origin security.debian.org
500 http://http.debian.net/debian stretch/non-free armhf Packages
release v=9.13,o=Debian,a=oldoldstable,n=stretch,l=Debian,c=non-free,b=armhf
origin http.debian.net
500 http://http.debian.net/debian stretch/contrib armhf Packages
release v=9.13,o=Debian,a=oldoldstable,n=stretch,l=Debian,c=contrib,b=armhf
origin http.debian.net
500 http://http.debian.net/debian stretch/main armhf Packages
release v=9.13,o=Debian,a=oldoldstable,n=stretch,l=Debian,c=main,b=armhf
origin http.debian.net
Pinned packages: