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If the Effects Paint Tool is not currently, active, tap and hold either the Paint Tool or the Spray Tool. A drawer should slide out; swipe to the Effects Paint Tool and release.
Tap the current effect in the tool's optons, then tap Dodge/Burn. You should be able to take it from there. Moving the slider more to the left burns, while moving more to the right dodges.
(Oddly, dodging and burning using this tool is not additive like Photoshop. In other words, if you try to dodge an area you've already dodged, the effect won't deepen.)