For converting IBIS models with .txt extension to .ibs format, the steps are as follows:
1)Save the .txt file in to any folder of C drive
2)Now make sure that "Hide Extensions for known file types" option available at Tools Menu->Folder Options->View Tab is unchecked.
3)Now open the .txt file and "Save As" it with .ibs extension, make sure you selected "All Files" option in "Save As Type" mirror.
This will result in a .ibs format created for .txt format in same folder.