Change font attribute

add this here to your child-theme functions.php:

add_filter( 'avf_google_heading_font',  'avia_add_heading_font');
function avia_add_heading_font($fonts)
{
$fonts['Ubuntu'] = 'Ubuntu:400,300,300italic,400italic,500,500italic,700,700italic';
return $fonts;
}

add_filter( 'avf_google_content_font',  'avia_add_content_font');
function avia_add_content_font($fonts)
{
$fonts['Ubuntu'] = 'Ubuntu:400,300,300italic,400italic,500,500italic,700,700italic';
return $fonts;
}

than you shoud have the choice on enfold options page – font to choose your Ubuntu (it is added at the end of the google fonts list on enfold )
400 is the normal font-weight so i think you should not have the need to change something than.
The rest is perhaps some quick.css (f.e. if you need the headings on 400 too – goto Enfold Options “Advanced Styling” and generate a rule for h1, h2, h3, h4, h5, h6 etc.)

you can try this first on quick css:

html * {
    font-family: "Ubuntu","HelveticaNeue","Helvetica Neue",Helvetica,Arial,sans-serif;
    font-weight: 300;
}

but there are a lot of inline css rules on enfold if you are working with alb (advanced layout editior)
you than have to define an additional rule for that!

f.e.

#top h1 strong, #top h2 strong, #top h3 strong, #top h4 strong, #top h5 strong, #top h6 strong {
    font-weight: 300 !important;
}

i do not know if it is possible to map the 300 weight with the “normal”