How do you upgrade to a smasher tank?

I found this upgrade diagram, but it doesn't explain how you upgrade to the smasher tank?

thanks in advance

Are you level 30 yet? Apparently you need to be.

From the Wiki:

The Smasher is one of the five basic upgrade choices from the Tank. Unlike other Tank Upgrades, it becomes selectable at Level 30. It may further upgrade into the Landmine, the Auto Smasher or the Spike.

It's considered one of the basic upgrades because it would be the first upgrade you make if you chose to wait until level 30. I'm assuming that the diagram above is tiered out to the corresponding levels you need to be to upgrade to that tank (meaning the Smasher tank is in the level 30 tier), but having never played the game, I can't say for sure.