Can I upgrade pricing plan for 1 month only?

Yes. You may upgrade your account for a month and downgrade to free as per your requirement. However, please note that JotForm is a subscription based service and the plan shall be renewed automatically. Since you want it only for one month, you may cancel the subscription after upgrading so that the account shall be scheduled to downgrade to free at the end of the subscription period.

Also, please note that the account limits shall be adjusted automatically when the account is upgraded/downgraded.

Hope this information helps!