EC2 instances use EBS volumes as storage. While AWS supports increasing volume size, size decrease is not supported.
create a new volume of smaller size, tag it
attach the new volume to the instance
create a filesystem and mount it
rsync -avzh /data to new disk
stop the instance
detach an old volume
detach a new volume
attach new volume and mount into the correct drive
start instance
If your instance have a lof of new data coming in and you want min donwtime, you can rsync all your data first, then stop your application and rsync data again.
The second start instance attempt can be removed if you're sure that the first will succeed.
You might want to check that your app is up after starting the instance.