For most forms

1. Log in as sysadmin in single user mode.
2. Go to Setup, Settings, then Company and Forms.
3. Type in the desired form's new Next Form Number.
    NOTE: If there's already a form with this number, select the next number to avoid errors in the future. 
4. Select Verify Number Sequence to have numbers sequentially entered.
5. Click OK.

For cheque forms

1. Log in as sysadmin in single user mode.
2. Go to Setup, Reports & Forms.
3. Click Cheques, then select the bank account.
4. Update the Next Cheque No field under Payment Cheque Settings and/or Payroll Cheque Settings.
   • If you selected the Use the same cheques for payment and payroll cheques option, update the Next Cheque No field, under Cheque Settings
      NOTE: If there's already a cheque with this number, select the next number to avoid errors in the future.