Sample code to modify the users  last viewed form id :

CRM saves the Id of the last form that a user viewed using the UserEntityUISettings entity, and any UserEntityUISettings record can be easily updated to set the Last Viewed form for a specific user:

//retrieve the user UI settings for a specific user and a specified entity:
QueryExpression query = new QueryExpression(UserEntityUISettings.EntityLogicalName);
query.Criteria.AddCondition("ownerid", ConditionOperator.Equal, userId);
query.Criteria.AddCondition("objecttypecode", ConditionOperator.Equal, entityObjectTypeCode);
EntityCollection UISettingsCollection = service.RetrieveMultiple(query);
if (UISettingsCollection.Entities.Count > 0)
{
//update the last viewed formId:
UserEntityUISettings settings = (UserEntityUISettings)UISettingsCollection[0];
settings.LastViewedFormXml = "<MRUForm><Form Type=\"Main\" Id=\"f5cfab6a-d4c2-4519-b68f-6e7485432e29\" /></MRUForm>";
service.Update(settings);
}