Upgrade SharePoint 2007 Application to SharePoint 2010
Launch Powershell with an account that access to the sharepoint environments.
If needed add the sharepoint snapin with the command:
Add-PsSnapin Microsoft.SharePoint.PowerShell
Once the snap-in is loaded type the following:
$webapp = Get-SPWebApplication http://sitename
foreach ($s in $webapp.sites)
{$s.VisualUpgradeWebs() }
Wait a few minutes and then the whole web application will be upgraded.
This is a one time upgrade and can not be downgraded.