Awk or Bash, Replacing string from specific column and line number to foreward

I want to substituted capital letter B with C in column 5 and from line 6 to the end of the file, need to keep the spaces as it is from my original input file as it is.

ATOM   1939  HG2 PRO A 125      35.681  32.906  38.437  1.00 43.59           H  
ATOM   1940  HG3 PRO A 125      34.593  33.765  37.652  1.00 41.79           H  
ATOM   1941  HD2 PRO A 125      37.364  34.075  37.624  1.00 43.38           H  
ATOM   1942  HD3 PRO A 125      36.333  34.312  36.415  1.00 41.29           H  
TER   
ATOM   1944  N   MET B  11      16.583  29.975  -4.306  1.00 51.32           N  
ATOM   1945  CA  MET B  11      15.542  30.263  -3.327  1.00 39.92           C  
ATOM   1946  C   MET B  11      16.146  30.366  -1.933  1.00 32.50           C  

I have read:

1. https://unix.stackexchange.com/questions/486840/replace-a-string-with-sed-from-specific-lines
2. https://unix.stackexchange.com/questions/70878/replacing-string-based-on-line-number
3. Sed replace pattern with line number

and my attempt is: awk 'NR == 6 && $ == 5, { sub(" B ", " C ") }'

>Solution :

This simple awk should help you in same. Written and tested in GNU awk.

awk '
FNR>=6 && match($0,/^(\S+[[:space:]]+)(\S+[[:space:]]+)(\S+[[:space:]]+)(\S+[[:space:]]+)(\S+)(.*)$/,arr) && arr[5]=="B"{
  $0=arr[1] arr[2] arr[3] arr[4] "C" arr[6]
}
1
'  Input_file

Using match function here to keep your spaces as it is even after substitution.