How can I adapt this program to a cash basis and just use a check register?

To adapt QuickBooks Online to a cash @jankay. Please see the steps below

In QuickBooks Online, go to the Settings or Gear icon, select Account and Settings, then navigate to the Advanced tab. Under the Accounting section, you can choose the accounting method as Cash.

See this screenshot below:

By following the steps, you should be able to modify your accounting method within your QuickBooks Online account. Let me know if you have other concerns.