Upgrade T1 Mex Auto
If you want the easiest way to upgrade, though the least controllable, you can choose for the Auto-Upgrade Toggle.
Simply activate the Toggle in the commands menu on any T2 construction unit. This unit will scan the entire map for T1 metal extractors to upgrade to T2.
TIP: You can still use this constructor to build other things. Once done it will continue searching for new upgrades.
PS. You will get notified if there are no more T1 extractors to upgrade, so you can then use the constructor for something else.