You can make a backup of all your eM Client accounts and data by copying the whole directory in which eM Client is storing its data.
For eM Client 6 I believe the default folder is C:\users<user name>\AppData\Roaming\eM Client.
I do not remember exactly the default location, because I have changed that in the settings of my eM Client.
Later, when you have a new computer/hard disk and re-installed eM Client, you should copy this backup-ed data over the default folder of eM Client (when eM Client is closed).
If you then start eM Client again, you will hopefully see all your accounts and data again.