If you read the file using TEXTREAD, you don't need to have the file truncated first, because you can tell TEXTREAD to skip a given number of header lines. Look at the doc for this function and the parameter named 'headerlines'. Example:
[node, X, Y, Z, modulus] = textread('myFile.txt', '%f %f %f %f %f', ...
'headerlines', 2) ;
If you really want to remove 2 lines from a file, you can build a solution around:
fid = fopen('myFile.txt', 'r') ; % Open source file.
fgetl(fid) ; % Read/discard line.
fgetl(fid) ; % Read/discard line.
buffer = fread(fid, Inf) ; % Read rest of the file.
fclose(fid)
fid = fopen('myFile_truncated.txt', 'w') ; % Open destination file.
fwrite(fid, buffer) ; % Save to file.
fclose(fid) ;
