You can change your My Flex/Mates plan, and tap save changes, and next month it will renew with the lower amount.
What you could do, is renew with an even lower amount next month, then change again for the following amount to be a bit more, and the even lower amount from next month will make up for the extra you purchased this month to even things out. This is as long as you're topping up before you're due to renew so your balances carryover.
I work for Vodafone, supporting our mobile and fixed customers, and volunteer my time on the community.
PM = Private Message. Click on my name to send me a message.