You need to remove the existing tile. Can't tile over tile.
Hi Linda, here's a post where someone painted the tile, which may be easier. https://www.hometalk.com/14911768/update-your-brass-fireplace-trim-and-outdated-fireplace-tile
And one about tiling over tile. https://www.hunker.com/13402160/how-to-tile-over-a-tile-fireplace
Here's a video tutorial on how to tile over tile: https://www.youtube.com/watch?v=0M5p4waRZ5s
This article might be helpful: https://www.tiledoctor.com/blog/tiling-over-existing-tile/