Update WordPress Plugin Programatically.

update_plugin_programatically.php

<?php
function replace_plugin() {
  // modify these variables with your new/old plugin values
  $plugin_slug = 'wp-reset/wp-reset.php';
  $plugin_zip = 'https://downloads.wordpress.org/plugin/wp-reset.latest-stable.zip';
  $old_plugin_slug = 'reset-wp/reset-wp.php';
   
  echo 'If things are not done in a minute <a href="plugins.php">click here to return to Plugins page</a><br><br>';
  echo 'Starting ...<br><br>';
   
  echo 'Check if new plugin is already installed - ';
  if ( is_plugin_installed( $plugin_slug ) ) {
    echo 'it\'s installed! Making sure it\'s the latest version.';
    upgrade_plugin( $plugin_slug );
    $installed = true;
  } else {
    echo 'it\'s not installed. Installing.';
    $installed = install_plugin( $plugin_zip );
  }
   
  if ( !is_wp_error( $installed ) && $installed ) {
    echo 'Activating new plugin.';
    $activate = activate_plugin( $plugin_slug );
     
    if ( is_null($activate) ) {
      echo '<br>Deactivating old plugin.<br>';
      deactivate_plugins( array( $old_plugin_slug ) );
       
      echo '<br>Done! Everything went smooth.';
    }
  } else {
    echo 'Could not install the new plugin.';
  }
}
   
function is_plugin_installed( $slug ) {
  if ( ! function_exists( 'get_plugins' ) ) {
    require_once ABSPATH . 'wp-admin/includes/plugin.php';
  }
  $all_plugins = get_plugins();
   
  if ( !empty( $all_plugins[$slug] ) ) {
    return true;
  } else {
    return false;
  }
}
 
function install_plugin( $plugin_zip ) {
  include_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
  wp_cache_flush();
   
  $upgrader = new Plugin_Upgrader();
  $installed = $upgrader->install( $plugin_zip );
 
  return $installed;
}
 
function upgrade_plugin( $plugin_slug ) {
  include_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
  wp_cache_flush();
   
  $upgrader = new Plugin_Upgrader();
  $upgraded = $upgrader->upgrade( $plugin_slug );
 
  return $upgraded;
}