If there are already some contacts uploaded you will just go into the app menu, tap Settings, tap Contacts, then tap Upload Contacts.
If you did not give the app permission when you initially installed it, to sync the contacts from your phone, follow these steps:
- Open the YouMail app and tap the Menu (☰) icon in the top left corner of the screen
- Tap on Contacts
- Click on Upload Contacts
- Then select Start Sync