Through the following code,
cols <- (heat.colors(255))
plot(raster_sum3, col=cols[round(seq(1,255,length.out=10))], legend=TRUE) # raster of the study area
plot(site_3003, add=TRUE) # site boundaries
I want to reverse the legend scale, in order to place lower values above and higher values in the bottom of the scale. Despite several researches, I did not find the appropriate syntax to reach my aim.