Assuming you have already setup the plugin project, which is a tricky task in itself, you would have to use the AssignRequest in the SDK.
https://msdn.microsoft.com/en-us/library/microsoft.crm.sdk.messages.assignrequest.aspx?f=255&MSPPError=-2147217396
// Create the Request Object and Set the Request Object's Properties
AssignRequest assign = new AssignRequest
{
Assignee = new EntityReference(SystemUser.EntityLogicalName,
_otherUserId),
Target = new EntityReference(Account.EntityLogicalName,
_accountId)
};
// Execute the Request
_service.Execute(assign);