Can I modify the Standard report in BC 16?

You can't.....

You need to create a new one.
You can copy it from the original, and modify yours.

As the others mention, you cannot do this as there isn't an easy way to modify a report. In theory, if you were creating a new report, you could add integration events that you could override to substitute data or you could provide alternate layouts in your report configuration. For base reports, however, I do not know of any that do either of these things.

Go into the "Microsoft_Base Application_<version>.app" file in your .alpackages folder of your project, find the report object that you wish to change, and copy that to a new file with a new object number and name (I usually just append "Custom" to the name).

If this is a report that has a report selections entry, you can simply change that selection to run the new report number. Otherwise you will need to have a codeunit that overrides ReportManagement OnAfterSubstituteReport or find all the page references to that report and modify the page actions:

[EventSubscriber(ObjectType::Codeunit, Codeunit::ReportManagement, 'OnAfterSubstituteReport', '', false, false)]
local procedure OnAfterSubstituteReport(ReportId: Integer; var NewReportID: Integer)
begin
if ReportId = Report::"Detail Trial Balance" then
NewReportID := REPORT::"Custom Detail Trial Balance";
end;