Change header´s title when using json_to_sheet

There’s no json_to_xlsx function, you probably mean json_to_sheet.

That being said, the easiest way is to change the worksheet after the fact. We’ll start from a simple example:

> var data = [{name:"Sheet", age: 12}, {name:"JS", age: 24}]
> var ws = XLSX.utils.json_to_sheet();
> ws
{ A2: { t: 's', v: 'Sheet' },
  B2: { t: 'n', v: 12 },
  A3: { t: 's', v: 'JS' },
  B3: { t: 'n', v: 24 },
  A1: { t: 's', v: 'name' },
  B1: { t: 's', v: 'age' },
  '!ref': 'A1:B3' }

The headers are automatically inserted in the first row (the A1/B1 from above). You can directly change the values. Let’s say “name” -> “Name”:

ws.A1.v = "Name";

If you want to loop through every header and make them uppercase:

var range = XLSX.utils.decode_range(ws['!ref']);
for(var C = range.s.c; C <= range.e.c; ++C) {
  var address = XLSX.utils.encode_col(C) + "1"; // <-- first row, column number C
  if(!ws[address]) continue;
  ws[address].v = ws[address].v.toUpperCase();
}

I don’t think that kind of formatting is possible, but to address your earlier question, the way I did it, since I also needed to change the order of the columns sometimes (although I know you can use the headers prop to do it), was to just map the keys of the JSON object to new keys:

newJson = oldJson.map(rec => {
  return {
    'Last Name': rec.lastName,
    'First Name': rec.firstName,
    ...
  }
}