Don't remove old plugin, just install it how you normally install new plugin. Because if you uninstall it then if that plugin has any data stored in it will also get removed. So, do not uninstall any plugin unless there is any problem with the plugin which cannot be fixed without removing it. Also if you have made any changes with the plugin in the codes then updating the plugin will replace those codes, so after updating you will have to make those changes again. So, in short just update the plugin without removing the old version. Also some plugins requires that you disable and enable the plugin after updating for database changes to take effect. So, check the update notes for that plugin and just disable and enable the plugin after updating if required by that plugin. Also you normally don't have to do it from ftp, you can do it from your Admin panel > Plugins > Install and it will automatically overwrite existing plugin.