BSP stands for Board Support Package, and BSP Editor is a GUI configuration tool available from Nios II SBT.

Settings related to make files and linker scripts are now more flexible than ever in the Nios II IDE. In addition, the related settings (System Library) have been migrated.

For how to use it, search for the keyword "BSP Editor" in the document "Getting Started with the Graphical User Interface" at the URL link below.
https://www.intel.co.jp/content/dam/altera-www/global/ja_JP/pdfs/literature/hb/nios2/n2sw_nii52017.pdf