You set the property of a field in a table:

If the property already exists, use:
CurrentDb.TableDefs("MyTable").Fields("MyField").Properties("Format") = ...

The function below sets the property if it exists, and creates and sets it
if it does not. Call it like this:
Call SetPropertyDAO(CurrentDb.TableDefs("MyTable").Fields("MyField"),
"Format", dbText, "Currency")

Function SetPropertyDAO(obj As Object, strPropertyName As String, _
intType As Integer, varValue As Variant, Optional strErrMsg As String) As
Boolean
On Error GoTo ErrHandler
   'Purpose:   Set a property for an object, creating if necessary.
   'Arguments: obj = the object whose property should be set.
   '           strPropertyName = the name of the property to set.
   '           intType = the type of property (needed for creating)
   '           varValue = the value to set this property to.
   '           strErrMsg = string to append any error message to.

   If HasProperty(obj, strPropertyName) Then
       obj.Properties(strPropertyName) = varValue
   Else
       obj.Properties.Append obj.CreateProperty(strPropertyName, intType,
varValue)
   End If
   SetPropertyDAO = True

ExitHandler:
   Exit Function

ErrHandler:
   strErrMsg = strErrMsg & obj.Name & "." & strPropertyName & _
" not set to " & varValue & ". Error " & Err.Number & " - " &
Err.Description & vbCrLf
   Resume ExitHandler
End Function

Public Function HasProperty(obj As Object, strPropName As String) As Boolean
   'Purpose:   Return true if the object has the property.
   Dim varDummy As Variant

   On Error Resume Next
   varDummy = obj.Properties(strPropName)
   HasProperty = (Err.Number = 0)
End Function