Well I have managed to understand this. Basically one must act with $\tau$ in every quark of the neutron wave function (https://wikimedia.org/api/rest_v1/media/math/render/svg/d01d80212401fe5d60053ac7dd8ffd0816c7e748 you can check it here)

Since we are comparing matrix elements, one can only use the $\tau+$. the isospin operator $\tau+$ will either transform a d quark to a u quark or give when acting on a u quark.

Once one acts in the neutron wave function, we project on the proton wave function (https://wikimedia.org/api/rest_v1/media/math/render/svg/d89ecb243a57bedbf3043da030615a1f733847c3)

Equation 3.30 we just act with the $\tau+$ operator on a neutron state, getting:

$\tau |n\rangle=-|p\rangle$

Don't forget the $1/2$ factor and the project this result on $|p\rangle$

Once everything is computed one can see that 3.29 and 3.30 wield the same result

Well I have managed to understand this. Basically one must act with $\tau$ in every quark of the neutron wave function (https://wikimedia.org/api/rest_v1/media/math/render/svg/d01d80212401fe5d60053ac7dd8ffd0816c7e748 you can check it here)

Since we are comparing matrix elements, one can only use the $\tau+$. the isospin operator $\tau+$ will either transform a d quark to a u quark or give 0 when acting on a u quark.

Once one acts in the neutron wave function, we project on the proton wave function (https://wikimedia.org/api/rest_v1/media/math/render/svg/d89ecb243a57bedbf3043da030615a1f733847c3)

Equation 3.30 we just act with the $\tau+$ operator on a neutron state, getting:

$\tau |n\rangle=-|p\rangle$

Don't forget the $1/2$ factor and the project this result on $|p\rangle$

Once everything is computed one can see that 3.29 and 3.30 wield the same result