Hi there. Just recently I have posted how you can workaround and change the data retention for Log Analytics in your Operations Management Suite (OMS) portal. You can check the blog post here.
Anyway, this change is now available directly in the Azure portal.
Login to your Azure subscription and navigate to your OMS workspace. Under Log Analytics blade go to the General section and click on Retention. Now you can change it up to 730 days.
And don’t forget to keep in mind that it comes with the extra cost!