You can change variables in a group of nodes just like how you would with any other nodes.
The first step is to access the enemy nodes in the group they're in.
var enemies = get_tree().get_nodes_in_group("your group name")
Then, in a for-loop, set each of the enemies speed like how you would with one of them.
var new_speed = 20.0
for(enemy in enemies):
enemy.speed = new_speed
Edit: BTW, this code would be found in maybe like the root node, but that's up to you as the programmer.