update current theme

Have you made changes to the theme’s original files?

yes, a developer also added a custom page template

Make a Child Theme
https://codex.www.ads-software.com/Child_Themes

Then copy all of the files you made changes to (apart from functions.php and style.css) and paste it into your Child Theme directory.

For your style.css file use a service like Diffchecker to check the difference between your theme’s original stylesheet and your modified stylesheet, to find what you’ve changed. Once you’ve found your changes, copy all of them and paste them into your Child Theme style.css file (at the bottom).

If you’ve modified functions.php, see this https://codex.www.ads-software.com/Child_Themes#Using_functions.php

i found another solution, thanks