To locate the access tokens for your workspace, go to the Developer Hub and then navigate to Your Apps.
Here you'll find a list of all the apps you've built, including the workspace the app is installed on and the production workspace's access token.
To find the access token, you'll need to locate the private app with the same name as your workspace's name. If this was migrated over during the update, it will have the phrase "Access Token App" at the end of its name.
For example, my production workspace was called CSMappe and the access token would be this private app in my developer hub:
Once you've located this, you'll want to click on the app's name. This will take you to the app's basic information view, like this 
Next, you'll want to locate your access-token details by clicking the Authentication link in the navigation bar.
Along with details regarding the scope of the token. (i.e. what data it can access and what it can do), you can also make updates to change the scope of your access token and more by clicking the Edit button, like you see below
If you have any questions about the Authorization page for an app (access token scopes, OAuth, or their use), you can find a lot of detail in our documentation guide about them here.