why can't I update my mac yosmite to high sierra?

Which exact MBP is it?

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)
• Don't use FireFox for these links…

https://support.apple.com/kb/sp765?locale=en_US

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

Hello, eugeniae67.

The macOS version you can upgrade to depends on the model and year of your Mac.

For details about your Mac model, click the Apple  icon at the top left of your screen and choose About This Mac.

These Mac models are compatible with macOS 10.13 High Sierra:

• MacBook (Late 2009 or newer)

• MacBook Pro (Mid 2010 or newer)

• MacBook Air (Late 2010 or newer)

• Mac mini (Mid 2010 or newer)

• iMac (Late 2009 or newer)

• Mac Pro (Mid 2010 or newer)

What happens when you try?

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS High Sierra App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS High Sierra.app in Applications, empty trash & restart, then a new copy of a more recent version can be downloaded…

Good news my friend, take care! :)