Retained APKs can only be deactivated, if you have newer update for roll-out to your previous release.
Go to your App releases tab under Release Management (Google Play Console) and scroll down you will find APKs to retain
and you can deactivate the previous outdated releases.
----- Update to my Answer, as of Oct 2020 -----
Simple answer for this is "Now you can't". With App Bundles it deactivates your previous release versions at the time of new version release.
Just for Demo Purpose (Step by Step)
Tried it with Internal Test Track holds same for Production Track as well.
Step 1. Go to Create Release for creating a new update release. Since, I already have uploaded the App Bundle, will go to Edit Release.
Step 2. You will see earlier version of your app. Click on the Retain button. By default, you don't need to take any action, since Google Play takes care of it, by deactivating it by default.
Step 3. Once you click Retain, you will see the button changes to Deactivate.
Step 4. But you will see an Error while trying to Review.
Step 5. Once you click on View Error Messages, you will see something like this.
Step 6. And if you scroll down, you will also notice a Red Icon Error, which means you can't surpass this error until and unless you Deactivate the earlier version, which you have Retained.
Step 7. Go Back and then Edit your Release Version.
Step 8. Click on Deactivate.
Step 9. After Deactivating you will notice that the Error is gone and also the Red Icon error is gone. Now you can review and publish your release version.
This has become invalid now after so many updates to Google Play.