Use this:
sudo apt update # Fetches the list of available updates
sudo apt upgrade # Installs some updates; does not remove packages
sudo apt full-upgrade # Installs updates; may also remove some packages, if needed
sudo apt autoremove # Removes any old packages that are no longer needed
Documentation about each apt option can be found in the the manpages for apt. These are also available by running man apt in your terminal.
Use of both upgrade and full-upgrade together is usually not needed, but it may help in some cases: see Debian documentation about Upgrades from Debian 9.