ASDF is an incredible version manager, to install/upgrade to latest version available of an language you can do the following steps:

asdf list-all <language>

this show all of the versions available to install, select the latest:

asdf install <language> x.y.z

and edit if you have a global ~/.tool-versions to use that version everywhere:

<languaje> x.y.z

real example

to upgrade to latest version of erlang (10/17/2019):

asdf list-all erlang
asdf install erlang 22.1
asdf global erlang 22.1