So the rangers companion adds your proficiency scores to certain rolls that it makes.
However all monsters have a base proficiency outlined in the Monster Manual that is based off their CR.
So my question is this, does your preficiency "replace" the beasts CR assigned proficiency, OR does it use both its own and your proficiencies added together (for example when rolling to hit)
The Ranger's proficiency bonus does not replace the beast's. You simply add the Ranger's proficiency to whatever number you see in the stats block on AC, saving throws (if any), skills, attack bonus and damage roll.
PbP: Out of the Abyss - Oorr - Aarakocra RangerPbP: Tomb of Annihilation - DM