how do i change a themes?

Go to “Site Admin” then “Appearance” tab on the left then Themes. Click a theme then click activate.

You need to read through the documentation on Themes here. It will tell you all you need to know.

can I use custom WordPress themes with the free hosted version www.ads-software.com or must we host our own WordPress server to do this?

Sorry, you can’t use custom WordPress themes on WordPress.com (free hosted).

i mean how do you upload new ones?