well..say Its happy line..from Em to F i will mute the song cause i bar that time...do i press with my fingers then comes the bar or just keep practiciing until im fast?

Just remember that the strumming should be constant and should never stop just so you can switch chords. I would just recommend switching from Em to F without even strumming but just by plucking each string one by one just so you can hear if it rings correctly. There is no need to rush anything. I always like to practice slowly so that I get everything close to perfection.

It happens to beginners, when they are not as comfortable switching chords, especially barre chords. I too have that problem from time to time when my fingers are not complete down for the barre and I already strummed. That's what happens and thus, the mute. With some more practice, then you can barre the strings quicker and you won't have that timing issue and will not mute the sound.

You don't bar and press the other string or vice versa... You press the whole chord at once.....
Practice makes perfect.

yeah practice makes perfect..!!

and press it harder....so the strings would make the sound..

yeah practice makes perfect..!!

and press it harder....so the strings would make the sound..