In the new trait, select "Replacement". In the "Trait to replace" field, choose "Ability Score Increase";
IMPORTANT: Add a modifier that DOES NOTHING, for example, adds a flat +0 to an ability score your homebrew species previously increased (if you skip this step, it will use the old ASI anyway);
Check "Customise my Origin" on a character;
In the species tab, select "Origin Manager" and check your new option (that does nothing);
How to update your homebrew species' ASI to work with 5.24