We earlier reported that Windows Phone 8.1 update 1 Build 8.10.14203.206 brought support for downloading software updates using MicroSD card to Windows Phone and enabling this feature depends on the Manufacturer.
Now one correction! It is Build 8.10.14219.341 that brings this feature and now one good news. Some Lumias with MicroSD card support that are updated to 8.10.14219.341 (either via developers preview or via Lumia Denim official update) have this feature enabled already. This is confirmed in an official FAQ reply.
Lumia software update OS version 8.10.14219.341 (formerly Denim), or above enables support for downloading phone software updates to a SD memory card. The supported devices list includes: Lumia 530/530 Dual SIM, 630/630 Dual SIM, 635, 636, 638, 730 Dual SIM, 735 and 830. The feature will be available for possible software updates coming after 8.10.14219.341.
Other Lumia models with a SD memory card slot, such as Lumia 520, 525, 620, 625, 720, 820, 1320 and 1520, do not support downloading updates to memory card.