how do i set default download location on lg l39

Unfortunately with your phone you cannot do that. At least not as it is.

If (and I say if because I can't find any info on how far forward your phone will allow upgrading to) your phone will allow you to upgrade the OS to "Marshmallow" then you 'may' be able to set it to download to the card rather than the phone. However, that also is only if the device can handle the larger/newer OS. Not all phones can, and even of those that can, not all can change to the download to card.

If by some fluke you can get "Marshmallow" installed on the phone you will then have to see if it will allow you to set the card as "Internal" rather than the standard "external". To do this they card should be blank. Why? Because if it isn't (and you don't back up what is on it) then all the data on the card will be erased when the card is formatted to work this way. Also the card will be unreadable by other devices.

The only other way I know of to try and make this work would be through rooting the phone, which is not something I could assist you with. For that I would suggest you visit the "Rooting, Jailbreaking & Unlocking" Forum