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.
P.S. If you are already using a theme it will not appear as a theme selection that you can make here > Appearance > Themes but all the other themes will be there to choose from.