I don't have Disney+ so I don't know if there's a direct way to do this. But this should work with about any subscription, I'd think.
Log into your account with whoever bills you for that channel. If you've subscribed directly with that channel, log into their website; if you subscribed through another service (Roku, Hulu, Sling, whatever) log into THAT website.
Unsubscribe from the channel. For most channels, this stops the automatic renewal at the end of the month, but your subscription continues til your last paid month runs out. (There could be some that will cancel immediately and give you a prorated refund, but I don't know of any.)
When your subscription ends, log back in and take out an annual subscription.
As a suggestion, when taking out the new subscription, do it directly with the channel. This way you can watch it on any supported device you may have, plus if any problems crop up there's no confusion as to whom you should be dealing with.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T