Site hacked and settings in payment module changed

It's not a security issue in HikaShop.
Once a hacker has gained access to your back end, he can do pretty much anything and change your HikaShop configuration.
A hacker can gain access to a website from many ways:
if you use a simple password, it could just figure it out. So make sure that you use a complex one.
he can use a hole in one of the components/modules/plugins/templates of your website or even joomla if it's not up to date. So you should not install extensions which are not widely used and if you don't use one, you should remove it. Finally, you should update joomla and the extensions on your website regularly.
Also, there are security plugins/components that you can use to increase the security of your website:
extensions.joomla.org/extensions/tools/security-tools