Beta-minus decay: a down quark becomes an up quark

In beta-minus decay a neutron turns into a proton and an electron is emitted:

$\text{n} \rightarrow \text{p} + \text{e}^- + \bar{\nu}_e$

That is the nucleon-level story. At the quark level, only one quark changes:

$\text{d} \rightarrow \text{u} + \text{e}^- + \bar{\nu}_e$

A down quark turns into an up quark. Because a neutron is $\text{u}\,\text{d}\,\text{d}$ and a proton is $\text{u}\,\text{u}\,\text{d}$, replacing one $\text{d}$ with a $\text{u}$ converts the neutron into a proton.

Check the charges in units of $e$, before and after:

Both sides give $-\tfrac{1}{3}$, so charge is conserved. The quark's charge rises from $-\tfrac{1}{3}$ to $+\tfrac{2}{3}$, so an electron of charge $-1$ must appear to keep the total at $-\tfrac{1}{3}$.

The antineutrino has no charge and almost no mass; it is needed to conserve energy and momentum (and lepton number), not charge.