In the Table view, when you click in a cell, it selects that record and all other records with the same value. See if you can use this to identify the records to delete. Scroll down through the table to check you've isolated the right records. Then visit the Data menu and delete the selected records.

Alternatively, use the query devices to filter in or out the data you want to delete. Use a Table view's Subset button to select either the filtered-in or filtered-out data as appropriate, then select all data in the view and visit the Data menu to delete selected records.

Does this help?

Alternatively, if you only want to remove headers from the start of the file, you can customise the number of rows to skip using "Customise data import" in the "Open file" dialog.