I've updated to magento 2.2.1. I want to change my account and create an account text to icons, I found the text in default.xml, so can some one let me know how to change the text to an icon
You should be able to do this in a translation file, like mentioned by Moin Malek.
Simply add a translation like
"My Account","< span class="icon">< /span>"
to your translation file.
The simple text should be replaced by your HTML markup (which obviously needs to be valid to work), that way you could "inject" an icon here without much hassle.