At 6th level, you gain a magical benefit based on the totem animal of your choice. You can choose the same animal you selected at 3rd level or a different one.
This means that it's possible for a Barbarian who selected the bear totem spirit for the Totem Spirit feature at level 3 (which grants resistance to all damage types except psychic while raging) to, e.g., select the eagle totem spirit at level 6 (which enhances their eyesight).
However, is it possible for a Totem Warrior barbarian, when they reach 6th level, to select another totem spirit option from the 3rd-level Totem Spirit Feature instead of selecting one of the options from the 6th-level Aspect of the Beast feature?
For instance, if a Totem Warrior barbarian has selected the bear totem at level 3, can they choose to gain the eagle totem's 3rd-level benefit (while raging, you can Dash as a bonus action) when they reach 6th-level?
As the 3rd-level Totem Spirit feature is "lower" in some terms, I wouldn't mind allowing it. This would create a 6th-level barbarian with extra resistances who is able to Dash as a bonus action, a pretty fast tank character (for example).
The same question also extends to the Totemic Attunement feature at 14th level. When a Totem Warrior barbarian hits level 14, can they select another (not yet selected) totem from the previous levels?