Edit a page/post with template not working

I did following steps :
1) If I create a new page by loading a template and save it.
Page created successfully !!

2) If I edit a page/post and load any other template, then it creates a new POST with that template, instead of loading that template into existing page/post.

Please suggest any solution..

Thanks

https://www.ads-software.com/plugins/simple-post-template/