There is no way to turn a non-RBF transaction to a RBF transaction. With changing any part of the transaction data, the signature becomes invalid and it must be signed again.


There are many nodes that have enabled full RBF and accept the replacement transaction even if the original transaction has not been flagged as RBF.
So, if you have an unconfirmed non-RBF transaction, you can broadcast a new transaction including the same input(s) and hope that the replacement transaction is included by a miner.


For making the replacement transaction using coinb.in, click on "New" at top of the page and then go to "Transaction".

- Paste your transaction ID and click on "Load".
- Add/remove any of addresses if you want to change the receiving addresses.
- For changing the transaction fee, you should change the value of outputs. (The transaction fee is the inputs value minus outputs value.)
- Click on "submit".
- Copy the unsigned raw transaction
- Click on "Sign" at top of the screen.
- Enter your private key and paste the unsinged raw transaction and click on "Submit".


For creating the replacement transaction, you can also use electrum.

- Create a new wallet using your private key.
- Let your wallet get synced and then disconnect your computer from the internet.
- Go to "console tab" and use this command to remove the transaction you want to be replaced from your wallet: wallet.adb.remove_transaction("TXID")
- Close electrum and open it again.
- Make the replacement transaction and export the raw transaction.


Now you have the singed raw transaction and you can broadcast it.
The problem now is that for broadcasting the transaction, you need a node that has enabled Full RBF and you should try different transaction broadcaster tools until you find a one allow you to broadcast your replacement transaction.
 
For broadcasting the unsigned transaction, you can also use electrum and change servers until you connect to a server which allows you to do so.