If you are familiar with Smarty syntax and are the admin, this can be changed by editing the template (tpl) file tiki-payment-single.tpl. Template files can be edited through the Tiki web interface by doing the following:
- Go to Admin Home > Look & Feel > Customization (tab) > Editing (section)
- Check both Tiki Template Viewing and Edit Templates and click Apply
- In that same section, click on the Edit Templates button that will now be visible
- Click on the tiki-payment-single.tpl link
I would highly recommend creating a custom theme (which can be a copy of a packaged theme) first if you haven't already. See http://doc.tiki.org/Customizing+Themes for how to do that.
I agree the look of these forms could use some improvement--please consider committing any changes you make that you think would be broadly useful to the project.