By default this new Replace-By-Fee feature is flagged as off, to turn it on I believe you can now just add this line to your bitcoin.conf file:
mempoolfullrbf=1
After version 0.5 the bitcoin.conf file actually doesn’t exist by default anymore, the default options are now set using command line parameters. However, you can still create and edit bitcoin.conf and add custom lines to it. The new path is shown here, and this command allows you to add new lines:
sudo nano ~/umbrel/app-data/bitcoin/data/bitcoin/bitcoin.conf
After you’ve added your new line(s) you can CTRL+X to exit, then Y to save, and then enter to skip renaming file.
Then you’ll need to restart Bitcoin for the change to take effect:
~/umbrel/scripts/app restart bitcoin
You shouldn’t have to but you may also restart your node via System > Restart or sudo reboot
to restart Bitcoin
As a disclaimer just in regards to the much discussed pros and cons on this feature and as mentioned in their post:
“Contributors to this project strongly recommend that merchants and services not accept unconfirmed transactions as final, and if they insist on doing so, to take the appropriate steps to ensure they have some recourse or plan for when their assumptions do not hold.”