you could do something with textscan and strncmp
for instance:
fid = fopen('cylds.bc','r+'); %open file for reading and writing
data=textscan(fid,'%s') %read file as string
data=data{1}; %for some reason maybe someone can explain you need to do this to get to the data
data(strncmp('PATCH',data,1)) = sprintf('%5s %5i %i','PATCH',2,1);
fprintf(fid,'%s',data); %write the data to the file
fclose(fid); %close file
Please note that I have not tested this so there may be a typo
