Isn't this like your third thread in SoPW about the same basic task? Could you have given some more detail this time about how those previous threads didn't solve your particular problem?
How about breaking things down: (1) process all files in a directory -- this just means get a suitable list of file names and do the same thing to each file; (2) in a given file, seek to a fixed position, read 10 characters (? or 10 bytes?) in order to get a string pattern that needs to be replaced with something else; (3) do a pattern replacement globally in the given file, and save the modified version of the file. Is that what you're trying to do?
There are a couple things you haven't described yet, which might be relevant:
Once you confirm that this script does the right thing, write another little script that takes as input a list of lines containing "filename 10_char_string". If there's something special about setting a replacement string for the "10_char_string", this script simply appends that replacement to the line and prints it out.
Once you confirm that the second script does the right thing, the third script is very simple: read the output of the second script, and for each line, open the file whose name is at the start of the line, slurp it into a single scalar variable, do a global regex substitution using the 2nd and 3rd tokens on the line, and write the resulting string to a new file.
Once you confirm that this last script does the right thing, you're done. Run something that prints a list of file names, one per line (e.g. "ls"), pipe its output to the "seek" script, pipe that one's output to the "set replacement string" script, and pipe that one's output to the "edit file data" script.
Each of those scripts is very short and simple. If you have trouble with any one of them, POST THE CODE THAT YOU TRIED for that step, together with a small sample of data that demonstrates the problem, and give us some idea about how the actual result differs from the intended result.