Software

in broad strokes, here is what you'll need to do:
an Update query is what you need to change the Tritium (wha-??) field value in A with the field value from the matching records in B
there is lots of good help and examples in Access Help.
i usually approach this problem by creating a Query and playing with it until it returns the records i want. you could possibly use the find unmatched wizard then saving the querying and modifying it to find matched instead. i think you will see when you try it.
then turn the Query into an Update query (do that with a button in the toolbar at the top in Query Design View)
also explore the Append and Create Table types of queries. maybe that will help you with your 'more records in b than a' question but not sure what you are asking...

write a query to update the values in a from b, based on the id numbers, then append the rest of the data from b to a that didn't append.

I agree with a combination of both answers 1 and 2. It's a good idea, as a matter of caution, to run a simple SELECT query based on the relationship of A.id and B.id and view/verify the results.
The next step is to turn that query into an UPDATE query that updates A.Tritium with values from B.Tritium.
If you want to consolodate the data from both A and B for purposes of a report you could always create a query based on a Full Join condition where all data and fields from both tables would be returned in the results. Note: NULLs will be returned in the fields that are not common in both tables.

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