How do I extend my milestone plan with an activation code?

Hello,

During the recent Black Friday sale I purchased an annual subscription of Milestones plan with a discount. The code was sent via email and I have it with me.

My current Milestones plan is schedule to expire in 5 days. I would like to use the purchased activation code but I couldn’t find anywhere to do that, not in the app nor in the website after I login.