Bit of a long one, but here are both:
pi@raspberrypi:~ $ sudo apt update
Err:1 http://archive.raspberrypi.org/debian stretch InRelease
Temporary failure resolving 'archive.raspberrypi.org'
Err:2 http://raspbian.raspberrypi.org/raspbian stretch InRelease
Temporary failure resolving 'raspbian.raspberrypi.org'
Reading package lists... Done
Building dependency tree
Reading state information... Done
55 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: Failed to fetch http://raspbian.raspberrypi.org/raspbian/dists/stretch/InRelease Temporary failure resolving 'raspbian.raspberrypi.org'
W: Failed to fetch http://archive.raspberrypi.org/debian/dists/stretch/InRelease Temporary failure resolving 'archive.raspberrypi.org'
W: Some index files failed to download. They have been ignored, or old ones used instead.
pi@raspberrypi:~ $ sudo apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages will be upgraded:
apt apt-transport-https apt-utils base-files bluez-firmware curl gnupg gnupg-agent gpgv libapt-inst2.0
libapt-pkg5.0 libc-bin libc-dev-bin libc-l10n libc6 libc6-dbg libc6-dev libcurl3 libcurl3-gnutls libpam-systemd
libperl5.24 libpolkit-agent-1-0 libpolkit-backend-1-0 libpolkit-gobject-1-0 libraspberrypi-bin libraspberrypi-dev
libraspberrypi-doc libraspberrypi0 libssl1.0.2 libssl1.1 libsystemd0 libudev1 libwbclient0 libxapian30 locales
multiarch-support openssh-client openssh-server openssh-sftp-server openssl perl perl-base perl-modules-5.24
policykit-1 python-rpi.gpio python3-six raspberrypi-bootloader raspberrypi-kernel raspberrypi-sys-mods
samba-common ssh systemd systemd-sysv tzdata udev
55 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 111 MB of archives.
After this operation, 711 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
(Lots of "temporary failure resolving" and "failure to fetch" errors)
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Should I try the commands at the bottom?