The LLVM bash script to update the apt repo's doesn't work on KDE neon.

It just says Distribution 'Neon' in version '20.04' is not supported by this script (Neon_20.04). so here's how to manually update to LLVM-13 or whatever verison.

If it's the first time add the key as described in https://apt.llvm.org/

wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -

Create a file in /etc/apt/sources.list.d/ named llvm-toolchain-focal-13.list with the following:

deb http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main
# deb-src http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main

If you're on a different distro change focal and the version as needed. If you want the source repos uncomment the deb-src file.

Then run sudo apt update and install sudo apt install clang-13 lldb-13 lld-13 clangd-13

Hope it helps someone.