Assuming your operating system is Windows, might work differently on other operating systems

1) Open File Explorer on your computer
2) Click on the bar containing a star followed by " > Quick access"
3) Click backspace to delete the text, and put in its place "%appdata%"
4) Click on the folder titled ".minecraft"
5) Click on the folder titled "saves"
6) Drag the folder in this zipped folder titled "The Netherflame Blade" into this "saves" folder
7) Launch Minecraft in version 1.21.4, and you should see the map in your worlds!