When a company is sourced from QuickBooks, all of the reports that we receive are formatted with respect to the company settings (Cash or Accrual) in Quickbooks.
You can change the method from Cash to Accrual, or from Accrual to Cash, by going to the company settings in QuickBooks and heading to the 'Accounting Method' property. After making the change in QuickBooks, you can update the company in Fathom (in Step 1 of Settings) to reflect your results in the new chosen method.
Right now, we're not able to provide cash-basis reporting from Xero. This functionality is on our development roadmap, and we are working to build this out on our end.
Feel free to send us a message at firstname.lastname@example.org to let us know you'd like to see this feature in Fathom!