In Pinyin, the tone changes for "yi" are not written. It's written in Pinyin as "yīyàng", but it's always pronoucned as "yíyàng". 

As far as I'm aware, this happens for all tones changes with "yi". For example 一个 is written as yīgè, but it's pronounced as "yígè". 

Here is the rule:

If "yī" comes before a 1st tone word, it changes to fourth tone "一般地说" is pronouced as yìbāndìshshuō and 一只狗 is pronounced yìzhīgǒu

If "yī" comes before a 2nd tone word, it changes to fourth tone. The last three characters of the phrase "这是怎么一回事" are pronounced "yìhuíshì"

If "yī" comes before a 3rd tone word, it changes to fourth tones 一小孩子 is pronounced "yìxiǎohaízi"

Here is where it changes, if "yī" comes before a fourth tone word, it changes to second tone. This is very common, because 一个, 一样 and 一件 are all like this. There are many other examples 一座城堡 etc.

This will be strange at first, but if you listen out for it when you listen to Chinese, it will become natural. 

Laurence