You’d likely want to do this in a staging site – many hosting support provides this service.
I’m not sure how Uncode theme works but if it’s using the default WordPress editor then all your content should be intact when you activate a new theme.
I am pretty new in that. Was working on uncode for lata 6 years but want to get more optimized website and was advised to get GP which I did and will start working on now. But don’t want to mess it up