There is no such thing as a theme reset or theme deactivation here. Changing themes is easy to do and no data is ever lost. You can go here Dashboard > Appearance > Themes and select a new theme and activate it. After you change your theme all you need to do is go here Dashboard > Appearance > Widgets > Inactive Widgets and re-install them where you want them to appear. The widget contents and settings will be the same as they were prior to changing themes.
To use the searchbox for theme features in order to narrow your search go here > http://theme.wordpress.com/