To convert time data with milliseconds to just h:mm:ss, you can use the custom time format in Excel. Here are the steps:

1. Select the cell(s) containing the time data with milliseconds that you want to convert.
2. Right-click on the selected cell(s) and choose "Format Cells" from the context menu.
3. In the Format Cells dialog box, select "Custom" from the Category list.
4. In the Type field, enter the following custom time format: h:mm:ss
5. Click "OK" to apply the custom time format to the selected cell(s).

Now, the time data with milliseconds will be displayed in the h:mm:ss format, with the milliseconds dropped off. For example, if a cell reads 12:34:55.789, it will be formatted to display as 12:34:55.

Note that this custom time format will only affect the display of the time data in the selected cell(s). The underlying value of the time data will still include the milliseconds, so if you use the time data in calculations, the milliseconds will still be taken into account.