In MATLAB, you can directly use logical indexing instead of writing a if-block
idx = hrs < 10;
hrs(idx) = hrs(idx) + 24;
In case, you want to see what is wrong with your code, following show how to use if-block with for-loop
for i = 1:numel(hrs)
if hrs(i) < 10
hrs(i) = hrs(i) + 24
else
hrs(i) = hrs(i);
end
end
