You can import your data into TickTick from other to-do apps, including Todoist, Wunderlist, Omnifocus, Toodledo and iCal.
Sign in to TickTick on the web.
Click the avatar in the upper-left corner of your TickTick homepage, then click "Settings" from the menu that appears.
Click "Backup" in the left panel, then click the "Import Backup" button on the right.
Tip: Importing from Wunderlist/Todoist can be done on TickTick mobile app via Settings - Security & Data.