There is no way of undoing something that you crafted before, but there are 3 different methods of “removing” or changing a given affix.
All 3 of these are random in some capacity and not 100% deterministic though.
Glyph of Chaos → while upgrading the tier of an affix it will be randomly (still weighted) rolled into another affix eligable for that slot (it will stay a prefix or suffix)
Rune of Removal → will remove a random affix
Glyph of Despair → has a chance will “seal” the affix, making room for crafting another affix, while still staying “sealed” on the item. A Item can only have one sealed affix, but a sealed affix does not count as prefix nor suffix, making room for another affix slot.
Was answering the first 2, but @Heavy already did.
As for the 3rd question, you shouldn’t worry about imbuing uniques until you reach empowered monoliths. Maybe not even then, to be honest. It has high RNG and it’s quite likely you won’t get much out of it.
Especially because you need to finish a dungeon boss to do it and that’s not always easy as you’re leveling up, especially when you’re new to the game and are not aware of the mechanics yet.