Where to find model for downgrade.Is that GSM or Global?
You can find the model number of your iPhone in Settings > General > About. There is an entry under "Model". This describes your phones model configuration, such as colour and memory size.

The model number you mention is the region and network configuration as per below table. So you have the Asia Pacific model.


iPhone 5s             Model Number
----------------------------------
GSM/North America     A1533
CDMA/Verizon          A1533
CDMA/China Telecom    A1533
CDMA/US/Japan         A1453
UK/Europe/Middle East A1457
China Unicom          A1528
Asia Pacific          A1530
China Mobile          A1518


What do you mean by "Global"? GSM is a telecommunications networking standard defining the protocols for what is commonly known as 2G.

Since you tagged your question as "iphone-5s", the networks it supports are


GSM
CDMA
3G
EVDO
HSPA+
LTE