Open the world in Creative mode or use commands with cheats enabled
Open the chat window
Use the `/fill` command to replace blocks in an area
Type `/fill x1 y1 z1 x2 y2 z2 block_name replace old_block_name`
Press Enter to run the command
Use `/fill x1 y1 z1 x2 y2 z2 block_name replace` to replace all blocks in the selected area with one block type
Use `/setblock x y z block_name` to replace a single block
Use a structure block or world editor for larger or more complex replacements
Use a command block if you want the replacement to happen automatically
Make sure the coordinates and block names are correct before running the command