How to edit file from WordPress menu

1) Open the WordPress Plugins menu page.

2) Click on "Edit" link under the "Booking Calendar" plugin.

3) On the right side of that page, please find specific file, like this:

booking/css/client.css

Or some other that you require. Click on that link.

Please note, if you do not see some specific file in list at the right side, so probably this file in some sub-folder (WordPress Edit plugin menu page do not show links to all files at once).
You will be need to edit URL in your browser and specify directly, what file to edit.

One restriction. (Booking Calendar Free version have name of plugin folder "booking", paid versions of Booking Calendar can have different names. For example Booking Calendar Personal version can have name of folder like this: booking.personal.7.0 (etc). So you will be need to change in URL this folder appropriately.

For example, if you need to edit this file: ../{Booking Calendar Folder}/core/admin/page-email-trash.php
Then you need to edit URL to this:
http://your-server.com/wp-admin/plugin-editor.php?file=booking/core/admin/page-email-trash.php

Important part here this path: ?file=booking/core/admin/page-email-trash.php
And in case, if you use some paid version, you will be need to replace folder name from "booking" to "booking.personal.7.0" or some other.

4) Make edit of this file.

5) Then click on "Update file" button to save changes.

6) Clear browser cache and test it.