Doogee Voyager2 DG310 - internal storage partition issues

Hi - my mum has the above phone, and up to now there haven't been any major problems. However, the last time I looked at it I noticed the internal storage was partitioned strangely, with the 8GB being split into 3GB-ish of "internal storage" and 5GB of "USB storage". This has now resulted in Facebook being unable to update due to a lack of space on the small partition. Is there a way to repartition the storage without rooting the device, or is there just some way to get around this issue? Thanks.

Edit: I may have the partition totals wrong.