I think this feature has been implemented since version 6.32

If you are using version 6.32 or above, you can look at mirc's menu bar, locate 'Help' to your far right hand side, drop it down to 'Check for Updates'...and voila. While it checks for update, a pop-up dialog will show to inform you if a latest update is available, along with an option that says 'Automatically check for updates,' which is located at the left corner. You can either tick it for automatic checking or do it manually.