Calc, file-open in another sheet

Hello!
I have imported sth to the first sheet in calc. (Sheet1)
Then I added another, ADDED, before Sheet2. (asheet)
I imported a document to sheet1. Then I opened asheet and wanted to import another document in there.
But, the second document appeared in SHEET1 again, INSTEAD of the document 1, which dissapeared from the sheet1.

Please, what to do?
Many thanks!

Please altair, a quick read to the inner help and the documentation https://wiki.documentfoundation.org/Documentation/Publications#LibreOffice_Calc_Guide, can help to know what is there and how to use it.

How are you importing, please explain step by step what are you doing?