Answered question
How you can use or activate it your upgrade?
You need to press “see details” in the my rewards program and choose the upgrade that you want to
Other answers
You need to click on it and pick the slot where to apply it
Since an upgrade boosts your monthly plan, your plan must include that service in the first place for that upgrade to work. Say for example you receive mobile data upgrade: your plan has to include mobile data for you to enjoy an upgrade consisting of mobile data.
Your plan showcases more than one available slot? Take advantage of the situation and assign an upgrade to each of those slots. Say for example you have an upgrade of 250 MB of mobile data, and another of 500 MB. Once these two upgrades have been activated on your plan, you’ll get to enjoy 750 MB (250+500) of extra mobile data each month.