If you received the “download and install” notification in the Windows Update settings indicating that the update is compatible with your device, but version 1903 doesn’t download, then the next best option is to use the Updates Assistant tool.
In the case that the Update Assistant doesn’t work, then you can use the Media Creation Tool to perform an in-place upgrade.
To answer the second question, you should read this answer.
Important: Before proceeding, it’s always recommended to create a full backup in case something goes wrong and you need to roll back. If you use these tools right now, you probably are going to install version 1909, instead. However, it doesn’t matter because 1909 is just a quality update for version 1903.