You can set a new allocation limit.
However, this won't lead to an immediate deallocation of resources. Instead the allocation limit is checked when new memory should be allocated. If the allocation limit is nearly reached or passed, then SAP HANA will try to free resources in order to fulfil the current memory request.
As freeing memory is a highly expensive operation and might not always be possible without restarting a process, I'd recommend to decrease the memory allocation limit and restart the instance.
That way there are no "surprises" about over-allocated memory and no delayed memory releases.