Before learning to play Higher Notes, it's important to already feel comfortable with these lower notes! **
Make sure you're using enough Air Support & your fingers are completely covering the note holes on the clarinet.
I recommend that you study these videos to learn how to play the higher notes:
Learn How To Play High Notes (1)Learn To Play High Notes (2)
Learn to Play High Notes (easy option)
** What if it's still difficult to play the low notes on the clarinet?
How to play Low G on the Clarinet
How to play Low F on the Clarinet
How to play Low E on the Clarinet