Upgrade mac 0S on mid 2010 macbook

Make sure you have Yosemite completely up to date before attempting to download High Sierra.

Also try clicking the link for the High Sierra installer using Safari or possibly a third party browser such as Vivaldi v3.7 which can be downloaded from links here since Firefox is known to have issues accessing the App Store):

https://vivaldi.com/download/archive/?platform=mac

If you still cannot download High Sierra, then you will need to first upgrade to macOS 10.11 El Capitan as that is available outside of the App Store from a link in this Apple article which also includes instructions for creating an optional macOS USB installer (recommended so you have more options if something goes wrong):

How to create a bootable installer for macOS - Apple Support

Make sure to read the article carefully as there are several steps to downloading and extracting the macOS 10.11 installer to the Applications folder.

You just need to check the App Store or the Software Update app or pane of System Preferences to see if there are any more updates available. Apple changed the location of the updates several times over the years so I don't recall where it is located for Yosemite.

Just follow the steps in the Apple article for creating a macOS 10.11 USB installer. You don't have to create the USB installer, but it does give you more options if something goes wrong during the upgrade. If you disconnect all other external drives except for the USB stick you will use for the installer, you should not have anything to worry about. If you get an error message, then just provide the exact command and error message (screenshot may be easiest, or copy & paste from the Terminal window to the "Additional Text" field accessed by the "Additional Text" icon which looks like a piece of paper).