To manually merge two contacts from your contacts list first, please select two or more contacts by tapping each contact photo, then tap the three vertical dots at the top right corner of the screen which opens a menu at the top of the app. From there you can select Merge to merge these contacts together.
To merge suggested duplicates:
Open Duplicates section from the Menu.
Select the contact you wish to merge and review. If you have more than two duplicates for the same contact, you can select which ones you want to merge.