To see how the InStr() function could work, you might try this query, using a COPY of Northwind's (table) Product (Products1).
The idea here is to find any ProductName containing "ee" and display the "ee" as "~~".
SELECT Left([ProductName],InStr([productname],"ee"

-1) & "~~" & Mid([ProductName],InStr([Productname],"ee"

+2) AS Expr1
FROM Products1
WHERE (((InStr([productname],"ee"

)>0));
If you wanted to eliminate any instances of "ee" entirely, the same code minus the & "~~" would do it.
To actually make the changes shown, you could use an update query based on ProductName where the criteria is InStr([ProductName], "ee"

> 0 and the Update To is
Left([ProductName],InStr([productname],"ee"

-1) & "~~" & Mid([ProductName],InStr([Productname],"ee"

+2)
Just remember to use a copy of your table, not the original.