@Publisher1953 - If you look on your server using FTP, does the /backend folder contains a index.php file ?
Anyway, let's try this:
1. download latest version of mailwizz on your desktop and unzip it.
2. Connect to your ftp and via ftp create a folder named newsletter at same level with the mailwizz folder, so basically you will be able to access it like: construction-marketing.org/newsletter
3. From your desktop, open the unzipped mailwizz folder, then go into the latest folder. Copy all that folder contents via FTP into your newsletter folder.
4. Point your browser to construction-marketing.org/newsletter/install and complete the install process, but make sure you use a new database. The old database will be used a bit later.
5. Login in the backend of the app.
6. Via FTP, open the file newsletter/apps/common/config/main-custom.php and change the database name into the old database name. You might need to change the username and password as well. If you need to, do it.
7. Go back to your browser and refresh it. This time, you should be prompted to update the application.
8. Follow the update instructions. Highly recommended to do the update from command line.
9. After the update completes, you should have a fully functional mailwizz application with your old data in it.
10. if everything fine, you can safely remove the old application files, otherwise post here.
Give it a try and let me know how it goes.
Thanks.