Watch the Day 3 online guitar lesson by Ravi from Learn Guitar in 21 Days
If you can't make the chord change from A to E in time with me, don't worry about it. Just leave the chord a little early so that we arrive together on the next chord - this also applies to all the other strum alongs in the course, and even when you are playing by yourself and counting out loud (which you should always do when learning and practicing).
Remember, don't pick up your first finger in this chord change, but rather, just slide it back and forth between the 2nd and 1st frets. With a little practice, you'll be playing along in no time.