The PHP version and its settings are under Account Manager → PHP Settings.

Pick a version from the PHP Version list and press Update Version. The change applies immediately. Before moving to a newer version check that your app supports it - current WordPress and its popular plugins run on the latest versions, older shops and scripts may be limited to older ones.
If the site stops working after the change, switch back - nothing is lost.
Add New Override changes individual PHP settings, for example:
Values cannot exceed your plan's limits.