Plugin Update current record

CRM2016

I have been writing a plugin which reads a record, does a bit of weird stuff and comes up with a 'result'. I'm using a postimage for the plugin.

The field I am trying to update is new_apacs18 text field multiline

Dim context AsIPluginExecutionContext =

DirectCast(serviceProvider.GetService(GetType(IPluginExecutionContext)), IPluginExecutionContext)

If context.PostEntityImages.Contains("PostTransSetInserted") AndAlsoTypeOf context.PostEntityImages("PostTransSetInserted") IsEntityThen

Dim PostTransSetInserted AsEntity = DirectCast(context.PostEntityImages("PostTransSetInserted"), Entity)

when I try  PostTransSetInserted.Attributes("new_apacs18file") =" some result" the plugin compiles ok but nothing gets posted back to the crm field.

any suggestions on where I am going wrong or a good tutorial to look at