Create an update action. During design time, you have to choose "This action will update the work item"

SourceCode.Workflow.Client.WorklistItem wi;
wi.ProcessInstance.DataFields["TestDF"].Value = "2";
wi.Actions["UpdateWorkItem"].Execute();

More info: http://help.k2.com/en/KB000307.aspx

I understand this but I want to change some of the variables while the process is active so that I dont have to delete the process and readd the process back.

The above code updates when the process is active.