If you are running OTG, you need to make sure you have Internet sharing enabled. See Sharing your Internet
For full details see: https://www.raspberrypi.org/documentation/raspbian/updating.md
The summary is:
First, update your system's package list by entering the following command:
sudo apt update
Next, upgrade all your installed packages to their latest versions with the following command:
sudo apt full-upgrade
Note that full-upgrade is used in preference to a simple upgrade, as it also picks up any dependency changes that may have been made.
It's worth checking with df -h that you have enough free disk space. The line with /dev/root mounted on / is the key line.
pi@raspberrypi:~ $ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 15G 1.4G 13G 10% /
devtmpfs 184M 0 184M 0% /dev
tmpfs 216M 0 216M 0% /dev/shm
tmpfs 216M 8.5M 208M 4% /run
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 216M 0 216M 0% /sys/fs/cgroup
/dev/mmcblk0p1 253M 47M 206M 19% /boot
tmpfs 44M 0 44M 0% /run/user/1000
pi@raspberrypi:~ $ uname -a
Linux raspberrypi 5.4.83+ #1379 Mon Dec 14 13:06:05 GMT 2020 armv6l GNU/Linux
pi@raspberrypi:~ $ cat /etc/os-release
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"