I assume you're talking about the challenge, having the leader fade behaviors between fleeing and following the mouse cursor?

If so, you want to calculate both a flee and an arrive velocity separately using the Steering.flee_from and Steering.arrive_to functions.

Then, you calculate a weight for the two behaviors based on some maximum distance past which you follow the target position instead of fleeing. A weight of 0 could represent fleeing completely, and a weight of 1 using only the arrive behavior.

Here's how I calculated the weight in the final assignment's source code:

var weight: float = clamp(max(to_target - FLEE_THRESHOLD, 0.0) / BEHAVIOR_TRANSITION_RADIUS, 0.0, 1.0)

I then used the Vector2 class's linear_interpolate method to interpolate between the two:

_velocity = flee_velocity.linear_interpolate(arrive_velocity, weight)