My computer won't update because of errors

My computer won't update and instead give me these errors. Please help if you can. My OS is Ubuntu 20.04.3 Focal Fossa

Hit:1 http://nz.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://nz.archive.ubuntu.com/ubuntu focal-updates InRelease              
Hit:3 http://nz.archive.ubuntu.com/ubuntu focal-backports InRelease            
Hit:4 http://security.ubuntu.com/ubuntu focal-security InRelease               
Hit:5 http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease 
Err:5 http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2763B0EE7709FE97
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2763B0EE7709FE97
W: Failed to fetch http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu/dists/focal/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2763B0EE7709FE97
W: Some index files failed to download. They have been ignored, or old ones used instead.

And when I try update from software updater.

You have to remove and re-add the mentioned PPA by using commands below:

sudo add-apt-repository -r ppa:kdenlive/kdenlive-stable
sudo add-apt-repository ppa:kdenlive/kdenlive-stable

and then install needed updates by

sudo apt-get update
sudo apt-get upgrade