You need to read this https://help.market.envato.com/hc/en-us/articles/202821460-Can-I-Get-A-Refund-
For obvious reasons by the nature of digital downloads refunds are only allowed in certain circumstances and direct exchanges almost never.
At best you would be refunded the cost of the original file to then buy the other.
Grill looks like a 5 star rated theme so would appear that there is not anything technically wrong with it, and the link above explains that a ‘change of mind’ is not grounds for a refund.
It has also been well over 30 days since purchase which is quite sometime to now ask to refund it
Most importantly of all - if it was not purchased by you then you have no chance of having it refunded and the original buyer would need to submit a request to support
They are the only people who may be able to offer some kind of resolution.