how can I updated to latest version from my MacBook version 10.13.6 year 2011
do I have to buy a new MacBook if I can't update the latest version
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There's no such thing as a 2011 MacBook. Apple Stopped making regular MacBooks in 2010.
Assuming you mean a MacBook Pro or Air and not a regular MacBook, 10.13.6 is the latest version a 2011 MacBook Pro or Air can run.
If you need a newer macOS version, then yes you would need to buy a newer computer. Otherwise you can continue to use High Sierra for as long as it works for you.