Updated UPGRADE.md [skip ci]

Closes #15828

framework/UPGRADE.md

@@ -5,23 +5,27 @@ This file contains the upgrade notes for Yii 2.0. These notes highlight changes
 could break your application when you upgrade Yii from one version to another.
 Even though we try to ensure backwards compabitilty (BC) as much as possible, sometimes
 it is not possible or very complicated to avoid it and still create a good solution to
-a problem. You may also want to check out the [versioning policy](https://github.com/yiisoft/yii2/blob/master/docs/internals/versions.md) for further details.
+a problem. You may also want to check out the [versioning policy](https://github.com/yiisoft/yii2/blob/master/docs/internals/versions.md)
+for further details.
 
 Upgrading in general is as simple as updating your dependency in your composer.json and
 running `composer update`. In a big application however there may be more things to consider,
 which are explained in the following.
 
-> Note: This document assumes you have composer [installed globally](http://www.yiiframework.com/doc-2.0/guide-start-installation.html#installing-composer) so that you can run the `composer` command. If you have a `composer.phar` file
+> Note: This document assumes you have composer [installed globally](http://www.yiiframework.com/doc-2.0/guide-start-installation.html#installing-composer)
-inside of your project you need to replace `composer` with `php composer.phar` in the following.
+so that you can run the `composer` command. If you have a `composer.phar` file inside of your project you need to
+replace `composer` with `php composer.phar` in the following.
 
 > Tip: Upgrading dependencies of a complex software project always comes at the risk of breaking something, so make sure
 you have a backup (you should be doing this anyway ;) ).
 
-Before upgrading, make sure you have a global installation of the latest version of the
+Before upgrading, make sure you have the latest stable version of Composer:
-[composer asset plugin](https://github.com/fxpio/composer-asset-plugin)
-as well as a stable version of Composer:
 
     composer self-update
+
+In case you use [composer asset plugin](https://github.com/fxpio/composer-asset-plugin) instead of currently recommended
+[asset-packagist.org](https://asset-packagist.org) to install Bower and NPM assets, make sure it is upgraded to the latest version as well:
+
     composer global require "fxp/composer-asset-plugin:^1.4.1" --no-plugins
 
 The simple way to upgrade Yii, for example to version 2.0.10 (replace this with the version you want) will be running `composer require`:
@@ -50,6 +54,7 @@ if you want to upgrade from version A to version C and there is
 version B between A and C, you need to follow the instructions
 for both A and B.
 
+
 Upgrade from Yii 2.0.13
 -----------------------