How could I read a file with header and then save the final file with header again?

How could I read a file with header and then save the final file with
header again?

I'd like to read file1 with given header and then implement some
mathematical operations on , for example, the x and Imag columns. Finally
I want to save the final file with header again.
How could I do it?
File1:
#ID ra dec x y Umag Bmag Vmag Rmag
Imag
1.0 53.141 -27.7967 3491.37 4060.43 23.1612 23.7058 23.0223 22.5351
22.1785
2.0 53.140 -27.7956 3496.66 4076.57 24.1362 24.8441 25.0093 24.7304
24.5864
3.0 53.142 -27.8008 3471.25 3997.29 24.1729 25.3841 25.3501 25.1032
25.0042
4.0 53.138 -27.7891 3527.16 4175.46 24.4685 26.1972 26.1785 25.5567
25.3188
5.0 53.146 -27.8085 3424.83 3880.66 24.425 25.1966 25.2755 24.9514
24.3456