What have you tried?

perl -pli.bak -e '$. != 1 and substr($_,18, 1) =~ tr/Y/H/;' filename

Here's how to read it:

• Run perl.
• Open filename for reading.
• Rename the input file filename.bak Open filename for output and select it as the default for STDOUT.
• Form an implicit while( <> ) { chomp; ... } continue { print "$_$/" } loop.
• If $. is 1 (first line of the file), skip to the next iteration.
• On all subsequent iterations $. will no longer be one; transliterate the 18th (zero-indexed) column from 'Y' to 'H'.

perlrun, perlintro, and perlop should contain 99% of what you need to learn to get a good start on the next one-liner yourself.

Update: Added play by play explanation.

Update2: Fixed issue where files aren't zero-indexed with respect to $. (Thanks Jim,Laurent_R)

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