Locate php.ini. For Debian this file is located in:
Search for the following configuration lines (in nano use CTRL+W for search) and replace the existing configuration values with acceptable values.
max_execution_time = 60 memory_limit = 128M post_max_size = 64M upload_max_filesize = 64M max_input_time = 60 file_uploads = On safe_mode = Off
Locate .htaccess file in the root of your wordpress directory and add or adjust:
php_value upload_max_filesize 64M php_value post_max_size 128M php_value memory_limit 256M php_value max_execution_time 300 php_value max_input_time 300
When you open the media library in WordPress and try to upload a file it should reflect the maximum configured upload file size: