TestID is a number between 1 and 70, but then it's 9945? I don't really understand the logic with getting your numbers, but you can just use if/then. Something along the lines of this

Dim X as Integer
If TestID=991 Then ' this would be a great spot to not having to right 100 lines of code if you have a method to this number
X=1102 'same as the line above
Else
X=ActivityID
End If

Currentdb.execute "Update InterestsT SET FinalID=" & X & " WHERE ID=" & ID

Answer to Docmd.RunSqlSQL
Sentyx
    DoCmd.RunSQL ("Update[Interests] Set[FinalId]= IIF([TestID] = 9921,21,[Finalid])")
The break down of the syntyx
[Interests]  The Table to update
[FinalId] The field to be updated
[TestId] the field to evaluate
9921 the item to be evulated
21 the value to be returned if evualation is true  [FinalId] would = 21
[FinalIDd] The value to return if evualation is false  [FinalId] = [FinalId]

Is this a one time thing or something you will need to do on going?

Thanks for the reply.  One time run. Renumbered about 100 items.  Like 1 and 4 became 56; 3 became 9.  
the iff sql worked great just had to copy and paste and change numbers then run a button update to run all 100 sql statements.
Thanks again for the reply.  I just couldn't get the docmd.runsql syntex correct.
Rex

OK so for the future - if this is a one time occurrence  -
I would use a simple update query.  Then just adjust for each case you need to change.

Creating a complex iif and debugging it probably took longer to do than just running an update.
On the other hand you did learn the ins and outs of making the complex iff so there is some benefit there.

My suggestion was going to be to make a real basic table with the "what it is" and the "what it needs to be" fields. then do a DLookup for the records looping through it.  IIF is really not meant to be nested that many levels deep.
I don't know where you are in your knowledge level but even a SELECT CASE statement would be easier to write one time that trying to nest 100 IIFs