tried code and it did not work.

I tried the code
define('WP_MEMORY_LIMIT', '64M');
and it did not work. It still rejects my video. Is there another way I can try to fix it? It shows the 8M maximum upload when I go to the section to browse for files on my computer. this is the exact message I get: Error Welcome vide…r MSTPHB.mp4
Welcome video for MSTPHB.mp4 exceeds the maximum upload size for this site.
Maximum upload file size: 8 MB.
please help me!!! =0)