This is a compressed archive created with 7-Zip . It is used to bundle multiple game files (like scripts, images, and fonts) into a single, smaller download.
Download and install 7-Zip or WinRAR. Right-click Nagisa_Eng.7z and select "Extract to [Folder Name]" .
Move the extracted contents from the .7z file into the game directory. When prompted, select "Replace files in the destination" .