Clone a server -Deb8


Dear community,
Do you know any way where I can clone a whole server (debian 8)? I have a 100 Go with my subscription but don’t know how to use it for backup (I know how to access it). I also have a local hard drive.
More context: I joined a company and have to keep the server up to date, it’s a mess, but it’s still working. I would like to upgrade it to Deb10 + new packages and apps but I am afraid to mess it up and break the dependencies. So I would like to keep an option of going backwards in case this doesn’t work.
Thanks for your feedback