It's easy to change the default donation frequency on one of your fundraising pages. For your core fundraising page, visit the Nonprofit Admin Portal Dashboard, then click "Basic Information" under "Edit Your Nonprofit's Core Profile". Then scroll down to "Set the Default Frequency" and change select "One-Time". You can follow the same process to change your page back at any point.
To change the default donation frequency for a campaign fundraising page, visit the Dashboard, find your campaign in "Recent Campaign Pages", and click "Edit Page". Click "Campaign Name, Description, and Goal". Scroll down to "Set the Default Frequency" and change it from "Monthly" to "One-Time".