go to index.html then remove this part
// Issue a warning if trying to preview an exported project on disk.
// Check for running exported on file protocol
if (window.location.protocol.substr(0, 4) === "file")
alert("Exported games won't work until you upload them. (When running on the file:/// protocol, browsers block many features from working for security reasons.)");
then SAVE. i hope i helped you
Just because you remove the notification doesn't mean that the game will work. It is the browsers own security that will proevent some features of the exported game from working, so removing the warning is not really the answer.
An analogy would be getting an 'oil warning' light appearing in your car so you decide to remove the bulb. The problem still exists, but you've just removed the warning.