On a document, to convert your work, whether your content is text, math, or a diagram, double-tap anywhere on your text block or object, or tap at the right of the screen, then select Convert all.
When converting text, the first conversion will be to the large, easily editable typed font. To confirm this conversion, simply double tap once again, or tap at the right of the screen, then select Finish.
For converting handwriting to digital text on a Note page or Note section, you can read more about this here:
Note: If you convert some text accidentally, tap in the top right to undo the conversion.