If you bought an annual subscription, you will have to change to a MONTHLY subscription (this is not an annual with monthly payments)
Annual vs Monthly https://forums.adobe.com/thread/1926342
Alternatively you might consider Photoshop Elements which you can get for a one-off payment.
.. or Gimp software, which is free.
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To emphasise, there is no plan which lets you say "I think I will use Photoshop this month, I will pay now for this month only". There are 3 plans for Photoshop only (as opposed to the whole suite of Creative Cloud apps). Prices are approximate.
$10/month (Photographer Plan, Photoshop + Lightroom). This is an annual plan, paid monthly - similar to a typical cellphone plan. You promise to pay for the year. Cancelling early still means you have to pay for the year.
$20/month (Photoshop only). Annual plan, paid monthly. More for less, a weird anomaly. Ignore it.
$30/month (Photoshop only). This is a true monthly plan, where you can cancel at any time. You have not committed to whole years, and don't have to pay for them. BUT, like all plans it auto-renews. So you have to cancel WITHIN THE MONTH to avoid paying for the next month, then take it out the next time you need it, and cancel again. You can leave the software installed, at least.