Put or modify php.ini in your root wordpress installation directory (eg. public_html/)
upload_max_filesize = 64M
post_max_size = 64M
max_execution_time = 300
Or put these inside functions.php theme file wp-content/themes/themes-folder-name/functions.php
@ini_set( 'upload_max_size' , '64M' );
@ini_set( 'post_max_size', '64M');
@ini_set( 'max_execution_time', '300' );
Some webhost allow changes via .htaccess in root directory
php_value upload_max_filesize 64M
php_value post_max_size 64M
php_value max_execution_time 300
php_value max_input_time 300