Pode ser um problema com o repositório de sublimetextos.
Procure na seção apt aqui: link
Tente executar isso antes de atualizar:
wget -qO - https://download.sublimetext.com/sublimehq-pub.gpg | sudo apt-key add -
sudo apt-get update
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://ppa.launchpad.net/webupd8team/sublime-text-3/ubuntu artful InRelease
Hit:3 http://dl.google.com/linux/chrome/deb stable Release
Hit:5 https://download.sublimetext.com apt/stable/ InRelease
Err:5 https://download.sublimetext.com apt/stable/ InRelease
Splitting up /var/lib/apt/lists/download.sublimetext.com_apt_stable_InRelease into data and signature failed
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: https://download.sublimetext.com apt/stable/ InRelease: Splitting up /var/lib/apt/lists/download.sublimetext.com_apt_stable_InRelease into data and signature failed
W: Failed to fetch https://download.sublimetext.com/apt/stable/InRelease Splitting up /var/lib/apt/lists/download.sublimetext.com_apt_stable_InRelease into data and signature failed
W: Some index files failed to download. They have been ignored, or old ones used instead.