This only works by phone. Be nice, but firm. Don’t be satisfied with their first answer – make them escalate you to the retention department. They’re often authorised to give much larger discounts because it’s cheaper for them to retain customers than to recruit new ones.
I’ve done this with SiriusXM. I bought a car, and I got a 6 month free trial. I kind of liked it, but not $24.95 a month worth. I called to cancel, and they gave me a much better price for 6 months. I went on my calendar, went forward 6 months, backed up 1 day, and put an entry to remind me to call back. 6 months later I called to cancel and got an even better price for a year. Did the same thing a year later, and got the same rate. The last time I called, they gave me a rate of just over $6 a month for good, so I don’t have to keep calling and negotiating. I know people hate on SiriusXM, but I absolutely get $6 worth of listening out of it every month.