I accidentally deleted some important tasks several hours ago, so, how I can undelete it?
also, does Todoist have history of changing, so I could restore some other data?
The only way to retrieve deleted tasks is by using the Backups feature. With Todoist Premium, one backup is generated for you per day, but you can create additional ones manually whenever you want. You can restore the data using the "Import from template" feature.
All details about how it works are shown in this video: http://www.youtube.com/watch?v=U96Nco_but4