To upgrade your plan, please head to Admin -> Account -> Plans and Billing.
There, you would find the different plans along with the provision to specify the number of agents, kindly select the "Choose the plan" option to proceed with your upgrade.
If you are on an Offline mode of payment (non-credit card method), you won't be able to edit your plan from the UI. In that case, please get in touch with Freshdesk Support (support@freshdesk.com) and your plan will be upgraded accordingly.