You can not.
you can customize the column headings in the list itself, right click on a heading and select customize – but you can not customize the drop down selector
Resolution for 'column headings issue: 'How I can customize the item list drop down menu?' available: Yes.
The column headings issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the column headings issue yourself by looking for a resolution described in the article. If it is a complex column headings issue or you are unable to solve the issue, you may contact us by clicking here or by using other column headings support options.
For more information about resolving issues related to column headings in Quickbooks/QBO or to get support for the issue 'How I can customize the item list drop down menu?', click the link below: