CRM 2011 PLUGIN to update another entity

My PLUGIN is firing on Entity A and in my code I am invoking a web service that returns an XML file with some attributes (attr1,attr2,attr3 etc ...) for Entity B including GUID. I need to update Entity B using the attributes I received from the web service. Can I use Service Context Class (SaveChanges) or what is the best way to accomplish my task please? I would appreciate it if you provide an example.

My PlugIn is firing on update in CRM 2011 On Premise.