If you are talking about making the SD card as the "Internal" rather than "External" storage, allowing downloads to the card directly, you can't actually do that on that particular phone, due to it running an older OS. Now if you can get it to run Marshmallow, you might, and I emphasize 'might' get it to allow you to set the card as internal.
Otherwise, the only way to do this would be to go and root the phone, and on that I can't advise you. You would need to visit the "Rooting, Jailbreaking & Unlocking" forum for that http://www.tomsguide.com/forum/id-3069142/set-default-storage-card-mention-working-lenovo-p70a-model.html.
But, if you can somehow upgrade the OS to Marshmallow, then the following is how to make a SD card "Internal"...