Ash here from the Shopify team. Welcome to the Shopify Community!
I'm sorry to hear of the issue you're running into while trying to change your payments information. I can totally appreciate how frustrating that must be. I'm happy to help out with resolving this! To ensure I have a full idea of what's going on, could you please reply back here and provide the following information:
Once I hear back with this additional information, we can work on the next steps together. Looking forward to hearing back!