How do I install minecraft 1.9 on mac?

Minecraft is now officially has its versions up to 1.8.8.

1.9 is not offically released yet, but snapshots came out. If you were referring to the snapshots, do the following.

In your launcher, click Edit Profile and scroll down the Use Version tab. Be sure to tick the Enable Experimental development verions (snapshots) before you go to the Use Verson tab. Click the latest snapshot, which is snapshot 15w33b (as of today).

You can now make a world and test out the upcoming 1.9 version of Minecraft!

Warning

After making your "1.9" world, do not enter the world with a 1.8 account as it may corrupt the world.

Everything you see in that snapshot is not confirmed. And may change during the process of making 1.9.