i have High Sierra 10.12.6,can i update to the newest version of garaband, like garageband 10?

That will not be possible, if as garageBand 10 is only sold at the AppStore. And you can only purchase GargeBand from the AppStore, if you have the most recent system version installed. You have to purchase GarageBand 10 (for free) from the AppStore on a Mac with a current system version to associate it with your AppleID.

Once GarageBand 10 has been associated with your APpleID and appears in your list of previous purchases, you can try to download an older version from the AppStore on your High Sierra mac. Open the APpStore there, sign in with your Apple ID, then click the fourth tab "Purchases" in the AppStore window and look, if GarageBand appears. Then click the "Download" arrow next to GarageBand.