How to pay off my remaining 0% interest for 4 months balance, instead of my direct debits?

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I’m trying to pay off my £360 0% interest for 4 months balance, before the 4 months is over, but I have an outstanding balance of £740 which I’m paying for monthly and expires in September 2021. Every time I make a payment, it comes off the monthly instalment payment, however I obviously want to pay the other one off, as it needs to be paid for, in 4 months! This is absolutely ridiculous logic, as this means it’s impossible for me to pay it off in 4 months, unless I can fork out over £1,000 in the next 4 months!
Surely there’s a way to pay the £360 balance off first? If not, this is an absolute disgrace of a system, as I can only afford to make those monthly instalments as they are, but don’t want to have to pay interest on the other balance, just because a pathetic system doesn’t give you an option to choose which one to pay off!
Please tell me I’m just being blind and PayPal aren’t as stupidly illogical as I’m currently assuming they are?!