Typically, you would use a form bound to the table and the user would scroll through the records to find the one they need to update. They can update the data in any of the controls and it will be saved to the underlying table. You can help the user out by adding a search function to the form which typically is done with a combo box (of the IDs) in the form header. When the user finds the ID they want and select it, the main form moves to that record. You can also add functionality to move to a new record if the ID is not there.
Of course, you could use an unbound form (i.e. not tied to a table), do the checking of whether the ID exists. You would have to provide controls on that form for the user to enter the new information (for a new ID) & then run the append query as you mentioned. For an existing ID, the user would not be able to see the existing information, so how would they know what information needs to be updated? Are you going to force them to retype all the information? Once you know which information needs to be updated, then you would run the update query. You would have to use code to determine whether the ID exists. That is fairly easy & would look like this
Dim mySQL as string
IF DCount("*", "tablename","ID=" & me.NameOfControlOnFormThatHoldsID)>0 THEN
'record exists, construct the update query
mySQL= "UPDATE tablename SET ..."
ELSE
'new record needed, construct the append query
mySQL= "INSERT INTO tablename..."
END IF
'execute the constructed query
currentDB.execute mySQL, dbfailonerror
