How to delete or exchange model files?

I am not satisfied with my uploaded files and want to exchange them with much better versions of them.
But, I can not do it because have not any tool or something to delete them, to change them...
Maybe I do not see something obvious, or to activate something on site, or what else to do...
I found some answers but they are very old and does not look like the actual view of the site.

Thanks!