ok,
$ mix phx.new -v
Phoenix v1.4.10
$ mix phx.new koko
$ cd koko/assets
$ npm outdated
Package Current Wanted Latest Location
copy-webpack-plugin 4.6.0 4.6.0 5.0.5 global
css-loader 2.1.1 2.1.1 3.2.1 global
mini-css-extract-plugin 0.4.5 0.4.5 0.8.0 global
optimize-css-assets-webpack-plugin 4.0.3 4.0.3 5.0.3 global
uglifyjs-webpack-plugin 1.3.0 1.3.0 2.2.0 global
webpack 4.4.0 4.4.0 4.41.2 global
webpack-cli 2.1.5 2.1.5 3.3.10 global
returns the list of outdated packages, then just update the versions of packages You want in packages.json and run npm install.
