How to impelement mapping point Curve25519 to weierstrass point?
I'm not sure what is your question, I'm assuming you are not understanding why the mapping gives that particular result...

In ECC, all computations are carried out in a finite field. For Curve25519, all computations are done modulo $p = 2^{255}-19$.

To compute $u + A/3$, the division by three is actually $A$ multiplied by the inverse of $3$ modulo $p$. This inverse is the value that, when multiplied by 3, given 1 modulo $p$. This inverse is 0x5555555555555555555555555555555555555555555555555555555555555549; you can check that if you multiply this by $3$ and compute the remainder of the division by $p$ it will give $1$. Carring out the rest of the computation (always reducing modulo $p$) will give the expected result 0x2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaad245a.

The modular inverse can be computed using the extended Euclidean algorithm or using Euler's theorem (which is computing the exponentiation by $p-2$).