how to update MacBook Pro (Retina, 13-inch, Early 2015) from high sierra?

Older versions are "put away" so that the innocent do not download them "by accident".

Click on the blue text, a link to the article that contains links to older versions of MacOS for download:

How to get old versions of macOS - Apple Support

If that is not working for you, explain how far you get and what error messages are seen..

back up all your files to an external drive, or they may get lost during the upgrade.

Links to downloads are available in this article:

How to get old versions of macOS - Apple Support

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