Searing Smite
As you hit the target, it takes an extra 1d6 Fire damage from the attack. At the start of each of its turns until the spell ends, the target takes 1d6 Fire damage and then makes a Constitution saving throw. On a failed save, the spell continues. On a successful save, the spell ends.
Using a Higher-Level Spell Slot. All the damage increases by 1d6 for each spell slot level above 1.
Searing Smite 5.5e 2024 vs. 5e 2014 edition differences
The 2024 version removes the ignition effect, requiring the target to take damage before saving, and ends the spell on a successful save rather than on a failed one. It also states that damage increases with higher spell slots, but clarifies that all damage increases by 1d6 per slot above 1st.