Do updating wifi firmware change my wifi setting as well? I have TP Link WR740N. Please help?

A major firmware upgrade could reset everything to defaults. That is why it is always best to do firmware updates from a wired connection. Changes to wireless setting should also be done from a wired connection, in case you made a typo and don't know what you set.

Some routers will lose their settings, some will not. It depends on the particular router and the reason why an updated firmware has been produced. If you update the firmware with a different firmware not produced by the router's manufacturer, then you probably will lose your settings.

It could, usually firmware updates only effect configurations if the feature change for specific configuration option. Shouldn't effect your login, so after the update you should be able to login and look around. You can also read the release notes for the update.

I have performed many firmware updates on mine. Never lost the settings.