Open Settings, then Subscription. It shows your plan and how many of this month's credits are left.
In the billing portal
Changing plan, updating your card, downloading invoices and cancelling are all done in the billing portal, which the Subscription page opens. Payments are handled by Stripe; AEOptimiz never sees your card number.
Buying more credits
Top-ups are bought on the Subscription page. See Credits and plans for the packs.
When credits run out
The assistant says you are out of credits and keeps your question, so it is waiting when you have topped up or upgraded. An article or rework that your balance cannot cover is refused before it starts, so nothing is half done.