Click on a tower and press R to rotate it (if available) and then click on the grass to purchase it. Click on already placed towers and decide between upgrading and deleting them. Deleting them gives you no value back so you need to place wisely.

This game got terribly corrupt, and I finally got to work on redoing everything that was once finished. So far everything is back to normal except for the first towers upgrades, but everything else is working now.