You can switch from your Personal to a Business plan in your account by clicking the Plans and Billing tab. Choose whether you prefer a monthly or yearly billing, enter your payment details and click Pay. Unused days of your previous subscription will be accumulated and recalculated: 2 days of the Personal plan = 1 day of the Business plan. Unfortunately, you cannot pay the difference or upgrade to the Business plan automatically.
If you are a Business plan subscriber and would like to add more websites to your account, you can extend your plan by selecting the Tilda Business 10 (10 websites), Business 15 (15 websites), Business 20 (20 websites), or Business 30 (30 websites) plans. Tilda offers both monthly or yearly billing cycles for all the plans available. The number of days remaining on the plan will be recalculated automatically once you upgrade your current plan.
Learn more on Pricing and payment: http://help.tilda.ws/subscription#personaltobusiness