Catechism of the Catholic Church §1125 — excerpt
For this reason no sacramental rite may be modified or manipulated at the will of the minister or the community. Even the supreme authority in the Church may not change the liturgy arbitrarily, but only in the obedience of faith and with religious respect for the mystery of the liturgy.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.