You cant do it in forge. You will have to access your AWS server.

(1) Create a snaptshot of the volume of your EC2 instace

To do this go to Elastic Block Store > Volume

and then right click of the instance you want to change the volume to

(2) Click “Modify Volume” and enter an amount and click Modify

that will change in the AWS console only and you will have to go to the terminal
to make actual changes in your server

(3) SSH to your server and issue command

sudo growpart /dev/xvda 1

Enter your sudo password you obtained when configuring server via Laravel forge

(4) Issue command sudo growpart /dev/xvda 1

(5) Then issue command sudo resize2fs /dev/xvda1

That should do it