If you can't import the demo content (the loading process never ends and the icon keeps spinning) then it's probably your server configuration that has low upload limits.
The easiest way to fix this is to simply increase those limits and below are the recommended option values:
- memory_limit 128M
- post_max_size 64M
- upload_max_filesize 64M
- max_execution_time 120
If you are not familiar with editing those values in your php.ini file then please contact your hosting/server provider. They should increase those values without in an instant without a problem.
wp-config.php Option:
You can also try to include the following value in your wp-config.php file:
define( 'WP_MEMORY_LIMIT', '128M' );
Not all hostings allow this or are suitable for this (particularly if on a shared hosting). Ask your hosting for more info.
https://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP