You should be able to play this song in less than a week, if you can't, please play songs from level 1
The best way to learn this song, or any other song, is to watch the video at least 10 times. Concentrate on the movement of the keys on the piano first and then look at the actual notes on the staff, feel the music and try to sing along with it.
Once you are familiar with the song, and how it sounds, then print the music score PDF below.
Now, with the actual music sheet on your hands, play the video a few more times and put your eyes on the paper, and read along with the music, listen to the song while looking at the music sheet.
Do that a few more times and with your index finger, pointing at the notes as the same time as you are listening to the music.
Now, count the measures in the song, in this song Frere Jacques, you have 16 measures. And you have six days to play the song. (The seventh day, you just relax and play the song for your friends or family.) So, if you have six days to play 16 measures, you have to learn almost 3 measures per day. Now look at the music and you'll see how simple it is, Right?