I cannot add comment to previous answer, so I decided to post my own.
As it's already been mentioned, you should use 'g' flag to replace all occurrences. But for
:s/X/Y/cg it will still replace only in current line. Reference from
4.2 Substitute *:substitute*
When [range] and [count] are omitted, replace in the
current line only.
You can specify range after colon symbol, like
:%s/X/Y/cg), to replace in the whole file. Or you can specify range from current line to document's end by
:.,$s/X/Y/cg). More about ranges you can see in