It's not that easy unfortunately, Sue, but I have been through it myself. As far as I know you can't change the linked account, and whatever you do, don't un-check the Show Cash in a Chequing Account as Reckon will transfer EVERY transaction from the linked account to the Investment account.
The work around I have used is to swap the descriptions of the 2 accounts. If the linked account is currently Account A and you want to switch it to Account B, then you need to change account A's details to that of Account B and then rename Account B as, say, Old Account A.
You may need to make an entry to adjust the balances of each account in Reckon to line up with your bank statements.