You can use SharePoint Designer 2010 to edit or modify a SharePoint Foundation 2010 site, this can be downloaded free of charge via the Microsoft website or by clicking the following links.
Before you can connect WSS Designer to your site you will need to login to your SharePoint site and go to Site Actions > Site Collection Administration > SharePoint Designer Settings and ensure the Enable SharePoint Designer is selected.
Note: Cobweb recommends before you make any changes to your site you make a backup of the content, otherwise any detrimental changes could be irreversible and costly to resolve as Cobweb may need to restore your content which is a chargeable service.
Open up SharePoint Designer 2010 and go to File > Open Site.
Type in the URL of the SharePoint site.
When prompted for the log in details, make sure the account you're using has the correct permissions and once loaded it will look similar to below.