While Microsoft has started releasing Windows 11, it is being done in a phased manner. It can take months for the new version to be actually available as a Windows update on your computer. On most systems, you will just get a message about being able to run Windows 11, without a specific timeframe mentioned.
In case your PC is selected for the current phase of the rollout, you can install it automatically from the Windows Update option in the settings. Otherwise, you will need to use the Windows 11 installation assistant.