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" rather than "External".
How to turn external SD card into internal storage on Marshmallow.
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in
there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.