This should help you (https://www.speedrun.com/smo/guide/qbeq4,
Credits goes to Smallant1):
HOW TO GET 1.0.0:
There are two methods currently that do not require "hacking" your
switch to obtain 1.0 and keep your save files for other games
METHOD 1 Requirements
-Physical Copy of SMO with serial number ending in 000
-Willingness to temporarily lose access to your SMO save data
-Nintendo Switch Online
- Backup the save data for all of your games on your switch using the Switch Online Cloud Feature
- Factory reset your Switch.
- Start up your Switch (DO NOT ENABLE INTERNET YET).
- Turn off auto-updates.
At this point, you can play SMO 1.0.0 and have no problems whatsoever,
but you don't have your save files back, so...
- Turn on internet.
- Redownload any games, and save data from the cloud. DO NOT download your SMO save data. If you download it then the game will not start
without forcing an update first.
You can now play Mario 1.0.0, but every time you try to boot it up it
will show a dialog box that will try to get you to update. You do NOT
have to do the update, but the box will still show up. Unfortunately,
any games that do not support the Cloud Save Backup will have their
save data lost. Keep this in mind. (Splatoon 2, Pokémon Let’s Go, NBA
2K19, FIFA 19, Dead Cells, Dark Souls Remastered)
METHOD 2 Requirements:
-Physical Copy of SMO with serial number ending in 000
-Willingness to lose your SMO save data
-A second Switch (Only necessary if you wish to keep save data for other games)
- Delete Mario save data from the profile(s) you want to "save" for later.
- Transfer the profile(s) onto another Switch (not necessary if you don't care about other games save data).
- Factory reset your Switch.
- Start up your Switch (DO NOT ENABLE INTERNET YET).
- Turn off auto-updates.
At this point, you can play SMO 1.0.0 and have no problems whatsoever,
but you don't have that profile yet, so...
- Turn on internet.
- Transfer your user profile back to your Switch from your friend's Switch.
You can now play Mario 1.0.0, but every time you try to boot it up it
will show a dialog box that will try to get you to update. You do NOT
have to do the update, but the box will still show up.