I have a iphone MC676LL/A which I believe is a 4. I am trying to change sim card but am in need of instructions

A search of that model number show it to be a Verizon iPhone 4. That device does not have a SIM card slot. There is no way to change it. That device will only work on the Verizon network. It does not have the capability to use a SIM or a GSM network.

Is there a sim card slot on the phone or does it have an embedded sim?

Ting, which sells sim cards for most models (Sprint on the CDMA side), does not list one for this model.

iPhone 4 Verizon Teardown - iFixit

https://everymac.com/systems/apple/iphone/specs/apple-iphone-4-verizon-cdma-spec s.html