Thank you for your help, it seems like I have to "restart Bitcoin Core with the -walletbroadcast=0 option and then you should be able to use "Abandon Transaction". in order to abandon those old transactions. But I have no idea how to restart with -walletbroadcast=0
The easiest way for me is to run Bitcoin Core with -zapwallettxes. It will abandon all unconfirmed transactions that the client has sent and you can script another transaction.
To do this, you have to run Bitcoin Core with the flag -zapwallettxes. You can do this via your command prompt. Assuming you didn't change the installation directory, these are the sample commands. Choose the one accordingly.
C:\Program Files\Bitcoin\bitcoin-qt.exe -zapwallettxes
C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe -zapwallettxes
Note that you should wait for a day or two and ensure that majority of the network has forgotten about your transaction. Else, your newly constructed transaction will not propagate well.