To edit these forms you will need to have a copy of MS InfoPath either 2007 or 2010.  Install this on the computer that you want to do the custom editing on.  I recommend that you make a copy of the forms you want to customize so you can edit and test prior to placing the custom form in the original location. 

To edit these forms you will want to navigate using File Explorer to the folder where the files reside.  Select the XSN file and right click on this file and select Design from the fly out menu.  This will open the form and you are now ready to complete your customization.

Once you customize your form and are ready to have these available on your projects, copy the edited forms to the original Templates location on your Newforma Project Center Server and the projects will automatically use the updated form.