Increase the PHP Time Limit
The PHP Time Limit refers to the amount of time (in seconds) that your site will spend on a single operation before timing out (to avoid server lockups). To prevent the importing process from timing out, the value needs to be increased. 300 (seconds) should be enough for processing the import file.
(Beginner) Ask your hosting company to increase it
The easiest way is to reach out to your hosting company and ask them to increase the max_execution_time to 300.
(Advanced) Increase the Max Execution Time via php.ini
Connect to your server via FTP and find or create the php.ini file. The following line must be added to it:
max_execution_time = 300;
(Advanced) Increase the Max Execution Time via .htaccess
Connect to your server via FTP and find the .htaccess file and add the following line:
php_value max_execution_time 300