Thanks V@no for your reply, highly appreciated
actually I was planin' to do it through CP, am gon' to manually update it
Thanks once again.
Update
I followed the steps with eyes wide open to not miss something, but I got this error
Warning: Division by zero in /****/******/********/includes/functions.php on line 586
and this line is the multi size download mod !! seems I will restore my 1.7.7 backup
Update II
Back to 1.7.7