Update DATA between two tables

So I have two tables; tblAppDetail and tblMainCustInfo. I need to update one column (Appt_Sidemark) between these two tables. tblApptDetail has field called: Appt_Sidemark and tblMainCustInfo has field called: Sidemark.
These two tables has CustID Linked.
When I try the following SQL, I am getting

1: Unexpected token:   in statement [update "tblApptDetail" set "tblApptDetail"."Appt_Sidemark" =]

Here is what I am running at TOOLS–>SQL
update “tblApptDetail” set “tblApptDetail”.“Appt_Sidemark” =
(select “tblMainCustInfo”.“Sidemark” from “tblMainCustInfo” where “tblApptDetail”.“CustID” = “tblMainCustInfo”.“CustID”)
where exists
(select “tblMainCustInfo”.“Sidemark” from “tblMainCustInfo” where “tblApptDetail”.“CustID” = “tblMainCustInfo”.“CustID”)

I know it is me…just not smart enough to figure it out by myself.
So your kind help is greatly appreciated.

The problem looks like it’s the WHERE EXISTS part. Looks like new HSQLDB supports IF EXISTS. Not sure if supported in 1.8. If that is problem, not helpful error message. Syntax like that is problematic in other database systems when using subqueries, even when it seems like it should work.

Where do I check my HSQLDB version.
I am running Windows 8.1 Pro / 64bit
LibreOffice Base
Version: 4.4.1.2
Build ID: 45e2de17089c24a1fa810c8f975a7171ba4cd432
Locale: en_US

I changed it to IF EXISTS and gave me error saying if in IF statement…So, I changed it back to WHERE EXISTS and took a change and WORKED … !!!
I tried it three different times and every time it worked just the way original code was.
Not sure why it didn’t run the other day; when I posted the message…

THANKS DOUG

You can use the macro at this link, modifying the string 'hsqldb' to the name of an actual LibreOffice Base filename on your system. For me on LibreOffice 4.4.2.2 it reported HSQLDB product version: 1.8.0.