Find and replace (overwriting) to the middle of an existing binary file

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I'm trying to open a binary file for replacing without erasing all the content. For example I can read my binary file and convert it to the struct as follows, what I want to do is that to replace data section with my own data:
if ~exist('filename', 'var')
error(['''' filename ''' does not exist']); end
% Open file
fclose all;
fid = fopen(filename);
if fid < 3; error 'Error while opening file'; end
% Read one property at the time
while ~feof(fid)
% Read field name (keyword) and array size
keyword = deblank(fread(fid, 8, 'uint8=>char')');
num = fread(fid, 1, 'int32=>double', 0, 'b');
% Read and interpret data type
conv = 'single=>double';
wsize = 4;
% Read data array, which may be split into several consecutive arrays
data = [];
remnum = num;
while remnum > 0
% Read array size
buflen = fread(fid, 1, 'int32=>double', 0, 'b');
bufnum = buflen / wsize;
% Read data and append to array
data = [data; fread(fid, bufnum, conv, 0, 'b')]; %<<========HERE========
remnum = remnum - bufnum;

Accepted Answer

Jeremy Hughes
Jeremy Hughes on 28 Mar 2021
fopen(filename,'a') % append mode
Then use fseek to go to where you want to overwrite.

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