This is 1st step 
Second … check your SSA template and make sure you only have Standard shipping enabled (NO economy, 2nd day, Expedited or Same Day). Set your preferred shipping method as USPS Ground Advantage (less than 1lb) and USPS Ground Advantage (1lb to 70lbs). DO NOT choose any other methods (if you need UPS or FedEx, they will still be available).
You have been shipping early if your AHT is 0 and 1 day handle time. To maintain a 2 day handle time, you will need to ship following a 2 day handle time schedule and never before.
For example … a store set to Monday thru Friday open with Saturday and Sunday closed
Order Day - Ship Day
Monday - Wednesday
Tuesday - Thursday
Wednesday - Friday
Thursday - following Monday
Friday - following Tuesday
Saturday - following Tuesday
Sunday - following Tuesday
Every ASIN / SKU will develop it’s own handle time so to get them all at 2 day handle time could take some time. AHT already has a pattern to them. Turning it off and back on does not change what it has already learned from your past shipping behavior.
With AHT and SSA turned on, shipping a day late does not hit your Late Shipment Rate. However, shipping a day late will mean you will see the Late Delivery Risk for slower shipping methods. Orders shipped with Late Delivery Risk do not have INR protection and can effect your OTDR. You will need a plan on how to execute this over a period of time.
Leave AHT on. We suggest starting with the 0 day ASIN /SKU orders and ship them as 2 day handle time (even though it will say Late Delivery Risk). If you can afford to buy a faster shipping method to avoid the Late Delivery Risk on these orders, then do so. Depending on how many ASINs / SKUs you are working with, try to keep track of which ones you have shipped as 2 day handle time as they will change to 1 day before they change to 2 day as AHT re-learns their shipping handle time. Track and watch you OTDR as you do this so you do not fall below the metric target rate of 90%.
Depending on the number of ASINs / SKUs you have and depending on the volume of each ASIN / SKU, the time it takes to re-train AHT will vary. Remember that all new listing should be done with the 2 day handle time. When a listing is new with no track record, AHT will use the item level handle time (which will be set at 2 day handle time).
The only way this works is by following the 2 day shipping schedule example above 100% of the time. No shipping early.
It can be done. We did it in about 4 months. However, we do not have a large amount of ASINs / SKUs and our volume isn’t massive (consistent but not massive).
Fast movers should transform relatively quickly. Medium movers might take some time. Slow movers might turn automatically after item level handle time is set if AHT doesn’t have anything on them.