The RGB histogram shows that this is a colorized grayscale image. The Pointer tool shows that in the HSV model, the only difference between pixels is the V (value/lightness), Hue and Saturation remaining the same.
For a quick job,
Colors>Colorify is your friend.
Possibly cleaner: add a transparent layer, set it to "color" mode (selector at top of layers list) and bucket fill with the color of your choice (knowing that the final result will be darker than the color you choose).
For extra control:
Color>Desaturate to convert it back to grayscale, then use
Color>Map>Gradient map to map the gray tones to a gradient (typically, a black-to-color one).
If you want more control,
Color>Desaturate to convert it to grayscale fol