The change building material changes depending on which set of controls you play on.
Old School Controls = LB/L1
Quick Builder Controls = Left analog stick pushed in
Combat Pro Controls = LT/L2
Builder Pro Controls = Left D-Pad
To know which controls you play on you need to go to settings and scroll across to the controller tab. It will then be highlighted on the left hand side which set of controls you play on.
Alternatively you can set your controller configuration to custom and set a custom button which you prefer to change building materials.
Should you change building materials?
If you didn’t know, the different materials have a different level of strength. In the order from weakest (Wood) to strongest (Metal). You’ll notice that a lot of the better players will switch to metal when being shot at because it takes a lot longer for the opponent to break through.
Whereas when you’re building to get onto buildings and up mountains. It’s better to use a weaker material and not waste any metal or brick you’ve built up. There’s a reason why metal is a lot harder to come across than brick and wood.
How to know when to switch?
The best way to find out when to switch is by watching the better players. If you watch videos of Ninja, Tfue and many of the other pro players and watch which building material they use in the different situations. This will give you a good understanding of what material to use and when.
Like any other game to get good at something you need to watch how other people play. And then apply those techniques to your own game.