If your solution has a gobal.json file, you can either delete it or update the sdk version over there to the latest.
At the point of this answer the latest .NET version is 6.0.0 with the sdk version as 6.0.100 (Latest sdk version could be found at the dotnet core site where you downloaded your sdk)
{
"sdk": {
"version": "6.0.100"
}
}
Where to confirm your sdk version from your download
After you have done that, ‘Unload’ and ‘Reload’ your project and the latest .Net Core target should show up in the dropdown.