To move up in Tiers, you need to meet the eligible criteria for each quarter. For example, if you are a Model in the rank bracket of 1-50 for more than 46 days during the quarter, and you upload between 9 and 14 videos, you will be in the Platinum Tier.
If you upload between 1 and 8 videos, you will be in the Gold Tier. Additionally, each video must be 7 minutes or longer in length. Please see the chart below for more details: