Am from South Sudan how can I change my payment method

is it possible for me to use airtime for calling

Contact your Carrier.

Am from South Sudan how can I change my payment method

Sorry is not possible, Apple does not have an App Store or iTunes Store for Sudan or South Sudan.

See -> Availability of Apple Media Services

Availability of Apple Media Services - Apple Support

Ask the carrier of your Ugandan SIM card if you can use it in South Sudan.

Check here: https://www.phonetravelwiz.com/buying-a-sim-card-in-south-africa-guide/

ONA -> https://www.mtn.co.ug/insight/east-africa-roaming/

Sim Card in Uganda -> https://www.traveltomtom.net/destinations/africa/uganda/sim-card-uganda