When are the objective pronouns le/les replaced with se?
The indirect object pronouns le & les change to se when proceding the direct object pronouns lo, la, los & las. I give it to him- Se lo doy. (can't be Le lo doy) She tells her mom the truth--She tells it to her. Se la dice.
Se is also used "impersonally" when it means "one" in general. One can buy milk here= Se puede comprar leche aquí.
As @toni mentioned the use of le/les or se depends on the type of the Object Pronouns (direct vs. indirect pronombres):
DO Pronouns: me, te, lo/la, nos, os, los/las
IO Pronounce: me, te, le, nos, os, les
When both are used in the same sentence, like here:
change the first pronoun to 'se' if both start with an 'l':
In such cases you should always try to avoid ambiguous meanings, like:
Here se can refer to Él, Ella or even Ellos or Ellas.
Additional examples (mind I'm not native and can be wrong):