I literally just did it 2 minutes ago.
My top tip for the day - buy the 3 month codes from CDKeys, currently on offer for £7.99.
Add them one at a time, and turn on automatic billing, as bizarrely it will give you the month free for a 3 month code too. Each time you redeem a code, turn back off the automatic billing, to get 4 months for each code. Rinse and repeat 9 times (it takes a while, as CDKeys will only sell you 2 at a time, but it's worth it!)
I just stacked 3 years for £72 (slightly less actually, as you can use the code PAYPAL5 on one of the transactions, and I cashed in my "CD coins" on another), then upgrade the whole lot for £10.99 - so around £83 total and I am sorted until June 2025!