I found out the solution thanks to Discord, I'm just posting it here, in case it helps others.
1. Go to the domain you want to cancel or delete & click Manage.
2. The actions you have there are: Change Auto Renewal, Renew Domain, Update Nameservers, Push to Client, Unparent and Reset Price. You don't need these for now, just click the Edit Service button in the lower right corner.
3. Now you have the actions Suspend, Cancel, Schedule Cancellation, Change Renew Date, Update Coupon.
4. You can choose to use the module or not. You could cancel the domain right away by selecting Cancel & clicking Save.
5. Afterwards the domain is placed in canceled domains, where you can delete it.