Upgrading

Open up composer.json and change the two instances of the version to the version you wish to upgrade to.

Check the versions of any modules installed under the require section and update as required.

The modules under require-dev may also be out of date, so you'll need to download the version of Magento you are upgrading to and check the composer.json to see if any newer versions have been added.

Now run:

composer update
magento setup:upgrade # php bin/magento setup:upgrade
magento setup:di:compile # php bin/magento setup:di:compile

Known issues

Composer problems

If you see something like the following

Can only install one of: magento/framework[101.0.0, 100.0.2].
Can only install one of: magento/framework[101.0.0, 100.0.3].
Can only install one of: magento/framework[101.0.0, 100.0.4].
Can only install one of: magento/framework[101.0.0, 100.0.5].

Then you'll need to double-check your composer.json and take a look at the modules/core version dependencies.

"Please re-run Magento compile command"

Annoyingly, it isn't clear which command to run, however this refers to running:

magento setup:di:compile # php bin/magento setup:di:compile