Hi Dave,
You need to stop the website on IIS, stop the application pool associated with that site also on iis. Then the DB file will become read by SQL Server Manager.
You need to then 'Attach' the database rather than attempt to open it directly. So you open SQL svr Manager connect, then open the folder called Databases and highlight it - right click and select attach - then click add (assuming your using 2008 version) then navigate to it.
Once you have it attached expand the database, and expand Tables, in there you will find dbo.version - right click and select 'Edit top 200 rows' then you will see the columns/rows - delete the one that has 6.1.3.
Close the open row, right click the top of the attached database, select tasks and detach, in the next window tick 'drop' and click OK.
You will have to then reset permissions in windows as editing the database in this way often resets permissions to just administrator. once done - start the iis website, then start the app pool.
then run the upgrade again by typing YOURDOMAIN.COM/install/install.aspx?mode=upgrade
This should work fine - it worked for me once I reset the permissions on all subfolders.
Cheers,
Andre