There are 3 ways that you can choose to increase the maximum upload file size in WordPress.

1. Use plugin (easy)

2. Contact hosting provider (medium)

Reach over to the customer support of your hosting provider and ask them to increase these limits for you
If you are still unable to import your file, the easiest way to figure out the problem is to request error logs from your hosting provider for your website. Examine the logs around the time when you tried to import the backup.
You will see what limit you have encountered and increasing it will solve the issue.

3. Do it yourself (hard)

Edit .htaccess file

php_value upload_max_filesize 128M
php_value post_max_size 128M
php_value memory_limit 256M
php_value max_execution_time 300
php_value max_input_time 300

Edit wp-config.php file

@ini_set( 'upload_max_filesize' , '128M' );
@ini_set( 'post_max_size', '128M');
@ini_set( 'memory_limit', '256M' );
@ini_set( 'max_execution_time', '300' );
@ini_set( 'max_input_time', '300' );

upload_max_filesize – set this to a value > than your backup
post_max_size – set this to a value > than your backup
memory_limit – set this to a value > than your backup
max_execution_time – set this to 0 (infinite)