If you want to upgrade or downgrade your plan, follow these steps:
1 - Go to your account settings:
click on your account icon on the top right corner, then click 'Manage your account'; OR
click the dropdown arrow next to your workspace name, then click 'Setting'; OR
click the settings button on the left side menu.
2 - Click 'Billing' on the top menu;
3 - Click the ... button to the right of your current plan's name, then click 'Change Plan'.
Now you just have to choose the plan, add your info and click 'Subscribe'.