The uploaded file exceeds the upload_max_filesize directive in php.ini

Here is 2 solutions.

A) Upload using FTP manager. It's can be any ftp manager for uploading files to your server. If you do not have FTP access, so then please contact support of your hosting. Also they can recommend to you FTP manager that can be good for your server to upload these files.

1) Please download the archive of the Booking Calendar to your desktop.
2) Extract it from archive. You have to get the "booking" folder.
3) Then upload this folder, using the FTP manager (like FAR or some other) to your server, to the folder

../wp-content/plugins/

4) Now open admin panel of your WordPress site. At Plugins menu, you have to see "Booking Calendar" plugin.
(If you was having previous free version, there, please deactivate it).
Click at the "Activate" link of your new paid version.

5) Please wait until message will show that the plugin is activated successfully. Now you can use the Booking Calendar plugin.

B) Or you can ask your serve administrator or support of hosting company to extend max file-size limit at your server.

Thank you.