[{"categories":[{"cat_name":"Web development","subcategories":[{"skills":["Angular","Css"],"subcat_name":"Frontend"},{"skills":["Nodejs","Express"],"subcat_name":"Backend"}]},{"cat_name":"Not web development"}]}]

db.collection.update({"categories.cat_name":"Web development"},{"$push":{"categories.$[catElem].subcategories.$[subcatElem].skills":"SASS"}},{"arrayFilters":[{"catElem.cat_name":"Web development"},{"subcatElem.subcat_name":"Frontend"}]})