Things are almost as simple if you have already installed, and need to force a redo. First, get rid of old search results by issuing rm BLDdir/tune/blas/level1/res/<pre><blas>_SUMM. Then, go to BLDdir/src/blas/level1/, and type :
rm Make_<pre><blas> make Make_<pre><blas> make <pre>liband that should do it.
So, to do that for the omnipresent DAXPY, on my PIII, I'd do:
cd BLDdir rm tune/blas/level1/res/dAXPY_SUMM. cd ATLAS/src/blas/level1/ rm Make_daxpy make Make_daxpy make dlib