Please how can I update WPLMS child theme in my website dashboard?
If you have made customization in child theme for making your requirements then if you update child theme then your all customization will be lost. In that case you can discuss with your developer and make the update manually to keep your customization safe.
But you can update parent theme to the latest version by downloading the latest version theme from Envato Market Downloads page and update manually using ftp.
Note: Better to keep a full backup before doing the update.