#Grafana has changed its signing key.

Just a friendly tip:
If you are on Debian/Ubuntu and apt update complains:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.grafana.com/oss/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9E439B102CF3C0C6
W: Failed to fetch https://packages.grafana.com/oss/deb/dists/stable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9E439B102CF3C0C6
W: Some index files failed to download. They have been ignored, or old ones used instead.

and assumed your grafana.list looks similar to mine:

deb [arch=amd64 signed-by=/usr/share/keyrings/grafana.gpg] https://packages.grafana.com/oss/deb stable main

then this will fix your problem:

wget -qO - https://apt.grafana.com/gpg.key | gpg --dearmor > /usr/share/keyrings/grafana.gpg

HTH and HAND