There are the steps which you can follow to demote any Header, let;s go through:
- Import excel files into power BI (load the data).
- Click on Home-> (right hand side) use first row as header-> use header as first row.
- Click on Transform Ribbon-> transpose.
- Right click on Column-> transform-> Trim.
- Transpose again.
- Again go click on home-> use first row as header-> Replace Value.
Using these steps you will be able to demote any header you want.
I hope these will be helpful for you.