I am quite new to wordpress, and haven’t found the answer in the help center –
I have a list of categories, but I want to take an existing category (that is already connected to several posts) and nest it under another category.
You should be able to do this in Posts -> Categories. Just find the category you wish to select a parent for, and then click on edit. Then select the Parent you want, and press save. This should still work even though there are posts already.
If it doesn’t, then you could create another category, with the parent already added, and then just add the posts to that category (though you’ll have to be careful about it turning into “category x-2”).