How do we get default emblem back?

Mugsy_: You can remove a purchased emblem by going back into the exterior / emblem 1 (or 2) section and clicking the little X in the top right hand corner of the currently placed emblem.
Important to note that this will remove the emblem and you will have to repurchase it to use it again in the future. So to recap, you pay for the emblem to be placed on the tank, rather than unlocking the artwork for future use.
That is, for the time being,
~ Mugsy_