How do I say "earlier"? (头先 in Cantonese)
"头先" means just now; a moment ago in Cantonese. It is the counterpart of 剛才，剛剛 in Mandarin.

You can use 头先 if few hours is considered a short time in the situation. 

If few hours is considered a long time in the situation, it is better to simply say: "几个钟头之前" (few hours ago)


  "你头先说的是对的” 


It is "頭先你講得啱" or "頭先你講得冇錯" in Cantonese 

The sentence in this context implies a very short time has passed--   within the length of a short conversation.