simple-plugin-update-example.php

function get_my_plugin_data(){
	// You'll likely replace this with some sort of API call home.
	$plugin = json_decode(
		json_encode(
			array(
				'new_version' => 89,
				'stable_version' => 89,
				'name' => 'My Fake Plugin',
				'slug' => 'my-fake-plugin',
				'url' => 'https://myfakeplugin.com',
				'version' => 998,
				'author' => '<a href="https://philjdrums.com">Phil Johnston</a>',
				'last_updated' => '2019-01-24 18:42:57',
				'requires'     => 1.0,
				'requires_php' => 7.3,
				'tested' => 900,
				'active_installs' => 50000,
				//'slug' => 'include-me-if-on-wp-org-repo',
				'homepage' => 'https://myfakeplugin.com',
				'donate_link' => 'https://myfakeplugin.com/donate',
				'rating' => 100, //percentage
				'num_ratings' => 19,
				//'ratings' => array( 5 => 10, 4 => 1, 3 => 7, 2 => 1, 1=> 0 ), //These are the number of reviews, not ratings. Only include if plugin is on wp.org as links are hardcoded.
				'package' => 'https://myfakeplugin.com/my-fake-plugin.zip',
				'download_link' => 'https://myfakeplugin.com',
				'sections' => array(
					'description' => wpautop( strip_tags( 'This plugin does this and that.', '<p><li><ul><ol><strong><a><em>' ) ),
					'installation' => wpautop( strip_tags( 'Here is how you install the plugin.', '<p><li><ul><ol><strong><a><em>' ) ),
					'faq'         => 'Frequently asked questions go here.',
					'changelog'   => html_entity_decode( 'This and that changed' ),
					'screenshots' => 'Screenshots go here',
					'reviews'     => 'Reviews go here',
				),
				'icons' => array(
					'2x' => 'https://philjdrums.com/wp-content/uploads/2019/01/Phil-J-Drums-1.png',
					'1x' => 'https://philjdrums.com/wp-content/uploads/2019/01/Phil-J-Drums-1.png',
					'svg' => 'https://philjdrums.com/wp-content/uploads/2019/01/Phil-J-Drums-1.png'
				),
				'banners' => array(
					'high' => 'https://philjdrums.com/wp-content/uploads/2019/01/Phil-J-Drums-1.png',
					'low' => 'https://philjdrums.com/wp-content/uploads/2019/01/Phil-J-Drums-1.png'
				),
				'contributors' => array(
					'johnstonphilip' => array(
						'display_name' => 'Phil Johnston',
						'profile' => '//profiles.wordpress.org/johnstonphilip',
						'avatar' => 'https://wordpress.org/grav-redirect.php?user=johnstonphilip',
					)
				),
			)
		), false, 4
	);

	// Format the contributors array the way WP core needs it.
	$formatted_contributors = array();
	foreach( (array)$plugin->contributors as $contributor => $contributor_info ) {
		$formatted_contributors[$contributor] = (array)$contributor_info;
	}

	// Format each part the way WP core needs it, as an object containing an array.
	$plugin->sections = (array)$plugin->sections;
	$plugin->icons = (array)$plugin->icons;
	$plugin->banners = (array)$plugin->banners;
	$plugin->contributors = (array)$formatted_contributors;

	// Set the name according to the plugin and name of the main file.
	$plugin->plugin = 'my-fake-plugin/my-fake-plugin.php';

	return $plugin;
}

function simple_plugin_updater( $update_plugins, $transient_name ) {

	$plugin = get_my_plugin_data();

	// Add this plugin to the list of plugins with update data being saved
	$update_plugins->response['my-fake-plugin/my-fake-plugin.php'] = $plugin;

	return $update_plugins;

}
add_filter( 'pre_set_site_transient_update_plugins', 'simple_plugin_updater', 10, 2 );