You can change your Direct Debit by logging into your InvestEngine account, then selecting your portfolio, then 'Options'. Then select 'monthly top-up'.
You first need to choose 'stop payment' before you can make a change and then input the new Direct Debit amount and/or collection date.
Please note for changes to come into effect as soon as possible, there needs to be a minimum of 5 working days clearing time from your current DD collection date. If you make any changes within 5 working days of this date, the amendments will come into effect the following month.