How do I equip/change mine or my units weapons?

When on the screen that explains the mission, you will see your team listed at the bottom, select the unit you wish to change their weapons (whether it be yourself or a friend), and you will be shown their current weapons. To change the weapon simply click the left or right arrows next to one of the current weapons to be shown the other weapons that they have learnt and stop when you arrive at the weapon you wish to equip, and then select the green tick.

Please note that your friends’ units can only use weapons that they have researched, the weapons you research are only for yourself.