For a quick dodge in Procreate select the eraser and reduce the opacity, while for a makeshift burn set the paint brush to black and use the opacity slider in the same way. But if you'd like something a bit more flexible and less destructive try this:
1. Create a layer above the one you want to colour dodge
2. In the layer menu click the little 'N' button on the right-hand side
3. Select lighten from the bottom tab bar and set type to, 'color dodge'
4. Click on the layer's thumbnail, select opacity
5. Slide your finger or stylus left across screen to lower opacity to 16% for example (you'll need something drawn on the selected layer for this to work)
6. Open layers again
7. Create another layer and repeat step 2, then select the darken tab and select 'color burn'
8. Repeat the opacity step 5
9. Now use these two layers to dodge and burn your image using the regular brushes
10. For subtle burning and dodging select the underneath colour and alter the shade slightly by adding or subtracting black or altering saturation, for a stronger effect go for black in the burn and white in the dodge (and to soften the effect use the smudge tool from the top bar), also use transparency of your strokes to control the amount of burn or dodge, but most of all experiment.