The convert/D function converts expressions involving derivatives in the diff notation to expressions in the D operator notation.

$f≔\mathrm{diff}\left(y\left(x\right)\,\mathrm{`\$`}\left(x\,2\right)\right)\:$

$\mathrm{convert}\left(f\,\mathrm{D}\right)$

${\mathrm{D}}^{\left(2\right)}\left(y\right)\left(x\right)$