CONCAT equivalent in MS Access

There are two concatenation operators available in Access: +; and &. They differ in how they deal with Null.

"foo" + Null returns Null

"foo" & Null returns "foo"

So if you want to update Null [My Column] fields to contain "Prefix " afterwards, use ...

SET [My Column] = "Prefix " & [My Column]

But if you prefer to leave it as Null, you could use the + operator instead ...

SET [My Column] = "Prefix " + [My Column]

However, in the second case, you could revise the WHERE clause to ignore rows where [My Column] contains Null.

WHERE [Different Column]='someValue' AND [My Column] Is Not Null

UPDATE myTable
SET [My Column] = "Prefix " & [My Column] 
WHERE [Different Column]='someValue';

As far as I am aware there is no CONCAT

You can use the & operator:

UPDATE myTable
    SET [My Column] = "Prefix " & [My Column]
    WHERE [Different Column]='someValue';