Can not update payment details. Changing credit card old expired

See the following Apple articles:

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

Add money to your Apple ID balance

Add money to your Apple ID balance

When I update my payment terms the system buffers and never concludes, later it shows the updated payment term but apple keeps telling me it does not work - driving me nuts!

the system buffers and never concludes,

Check here see -> About iOS performance on your iPhone, iPad, or iPod touch

About iOS performance on your iPhone, iPad, or iPod touch - Apple Support

This answer is not helpful at all.