OK so the first thing you need to do is read sheet1 into a datatable (with headers)
You can then use a for each loop with datatable. Columns and datatype is datacolumn.
This will go through each column.
Use an if statement inside the for each to check if the cell on the first row is empty of not.
Something like string.isnullorwhitespace(dt.rows(0).item(item).tostring)
If it returns true you then have the column you need to update. You can use an assign to get the column number using item.ordinal
Item is the datacolumn used inside the for each.
Then take a look at this post to convert that number into the relevant column letter
Another option which may be simpler is to just read the whole datatable, update the data inside the datatable in UiPath then overwrite the whole sheet1 starting at range a1.