Reset your website

Hello,

I want to completely change my website template, as well as categories and pages … Also, I found a template that suits me but when I apply it to replace my current site, it adapts and a certain number of pages and other functionalities disappear because I did not have them on my old model … I therefore wish to reset my site to zero and apply my new model so that it appears with all of its characteristics by default.
How to do ?

By the way, if I do “delete all content” will that delete the members?

Best regards,
Mihai Guyard
mihai.guyard.orange.fr