Changing Theme

[ Moderator note: moved to Fixing WordPress. Please do not use Developing With WordPress for these topics. ]

Hello Everyone

Has anyone or does anyone know if its at all possible to redesign a wordpress website in the background using a different theme without having any downtime?

Would i need to use a childs theme? or would i need to design it in a whole new website space?

I just started working as a marketing manager and the website i have currently is hosted by the people who designed it last time so i have no access to the backend of the server just the backend of wordpress

any advice or help would be much appreciated

Thanks,
Rob