You mean, you did not search ESO's support pages?
https://help.elderscrollsonline.com/app/answers/detail/a_id/4863/kw/upgrade%20imperial%20edition
CAN I UPGRADE MY PRE-ORDER FROM A STANDARDEDITION TO AN IMPERIAL EDITIONWHENEVER I WANT?
Updated 02/25/2014 05:35 AM Published 01/28/2014 09:53 AM
If players have pre-purchased through our official http://store.elderscrollsonline.com/en/and would like to upgrade to the Imperial Edition, we are planning on having an easy-to-use upgrade service available at launch. For further queries on this, please contact our dedicated http://help.elderscrollsonline.com/app/ask team.
Players who would like to upgrade their pre-orders should visit their retailers to discuss their pre-order options.
Soo.. it depends on where you bought it.
The other answer is:
https://help.elderscrollsonline.com/app/answers/detail/a_id/4729/kw/upgrade%20imperial%20edition
CAN I UPGRADE TO THE DIGITAL IMPERIAL EDITION?
Updated 03/04/2014 06:36 AM Published 01/23/2014 04:40 PM
Starting April 4th, 2014 all customers who pre-purchased the Digital Standard Edition can upgrade to the Digital Imperial Edition by visiting The Elder Scrolls Online Store (https://store.elderscrollsonline.com). This will allow them to receive the exclusive collection of in-game rewards available from the Digital Imperial Edition. This upgrade service will be available from April 4th, 2014.
If players have purchased a physical copy of the Standard Edition but wish to upgrade to the Digital Imperial Edition, our upgrade service is also there to assist them.