Manipulating coordinate data in an array

I have an array of data, the second dimension of which is landmark coordinates derived from paired images. I need to reflect (by multiply -1) either the y or z axis coordinates for a certain number (but not all) of my images to run further morphometric analyses (allows me to superimpose left and right sided shapes). I suspect this is relatively easy, but I'm new to R and haven't been able to figure it out. Example of what I've tried:

df$coordinates [ , 2, 1:10] *-1 # an attempt to reflect the second coordinate colunn in images 1 through 10. This just extracts the data from the array.

I apologise for my ignorance and appreciate any assistance.