Don't be afraid, it is not a big deal.
Please open a terminal and invoke the folloving command:
sudo apt update
You can ignore the lot of output, but look at the last few lines.
If you can see "Hash Sum Mismatch" in the messages, then you can make some housekeeping by command
sudo rm -rf /var/lib/apt/lists/*
and then repeat
sudo apt update.
You can see something like this:
"W: Failed to fetch http://ppa.launchpad.net/....."
Please turn off this ppa-repository in your repo list, then repeat "apt update",
Okay brother, thanks.