All you need is the Techtree - Upgrades To field.
Object Editor > Units Tab > Select any unit > Techtree - Upgrades To, edit this.
Let's say you have 10 Levels of Tower, right ?
So you have to create 10 custom tower.
After that, at the first tower, set the Techtree - Upgrades To to Tower Level 2
At Tower Level 2, set it to Tower Level 3, and so on.