Yes. Wello Solutions is a pay-as-you-go service and you can off course switch between yearly and monthly. Just go to the Subscription page to do so.
You can switch at any time except for one special case: downgrading from yearly to monthly is only possible as from 3 months before the end of the year. Take into account that not all plans offer a monthly subscription.
The change in recurrence always takes effect at the end of your current subscription period.