how to upgrade wordpress core and plugins

• download new WordPress files and delete htaccess,wp-content,wp-config.php from it
• the go to the old WordPress site remove all files and folders except wp-config.php,.htaccess,wp-content from it
• the copy and past it

same in the case of the plugin 

backup plugin and update the news plugin check for any error or template broken in the site

then move to next plugin

reference

https://codex.wordpress.org/Managing_Plugins

https://www.wordfence.com/learn/how-to-manually-upgrade-wordpress-themes-and-plugins/