A bearer token can only be associated with a single account at any time.
In order to change the association of your bearer token with a different account, you must perform a masquerade operation. Then you can perform API operations on the new account id. This only works if the authentication session user has access to the account id you specify.
How do I know if my user has access to multiple accounts?
Sign in to your account admin app. If you have access to multiple accounts, you’ll be presented with a list of accounts to choose from right after signing in successfully. If you’re already signed in, look for the account dropdown in the top left of the app. You’ll see multiple accounts in the list. If you do not see any other accounts, then your user only has access to your one account.