If the avatar already has a separate folder for pc and quest, then the avatar is already quest compatible; you just have to make a different unity project, change the platform in the vrc upload panel (the gray box at the bottom that is labeled platform) and next to it (it usually says windows), if you click it; it will give you option between Windows (PC), Android (Quest), or IOS (iPhone), to upload it. But, if you mean conversion like the avatar ISNT already quest compatible then there's a MANY tutorial on the forums and online about it. I can provide those links if you'd like.