How do I download the newer version of Minecraft?

You have to buy the newer version

The newer version of the game which is simply titled "Minecraft," also known as Bedrock Edition or the Windows 10 version, is basically an entirely new game compared the "Xbox One Edition." This version of the game has not been supported in sometime and
is equivalent to basically the Xbox 360 version of the game. You'll have to buy the 'newer' version. It has been out for quite some time. When this version initially launched there was a promotion to get it for free but, that is no longer the case.

Yes the newer Version you have to buy ,

Hi there,

just to be sure are you using the 'correct' version of Minecraft on the Switch, since there seem to be two versions. the 'older' Minecraft for Switch version and the 'new' Minecraft. Owners of the older version should be able to download the newer version
for free.

If you have the new version, have you tried the exact steps mentioned at

'Now that I have a Microsoft account, how do I sign in on my Switch?' on this page:

https://support.xbox.com/games/game-titles/minecraft-switch-edition-faq

That link will also have info on the two versions and other frequently asked questions.

Let us know the results

sounds like an issue with the version of minecraft.

It sounds like the realm was created on a newer version of minecraft than what you have installed. Just check for an update to minecraft.