If your implementation already provides ASDF 3 or later (and it should), but you want a more recent ASDF version than your implementation provides, then you just need to ensure the more recent ASDF is installed in a configured path, like any other system. We recommend you download an official tarball or checkout a release from git into ~/common-lisp/asdf/. (see Configuring ASDF to find your systems).
Once the source code for ASDF is installed,
you don’t need any extra step to load it beyond the usual
ASDF 3 will automatically look whether an updated version of itself is available
amongst the regularly configured systems, before it compiles anything else.
If your implementation fails to provide ASDF 3 or later, see Replacing your implementation's ASDF.