Learn Dutch: Je kunt de helderheid en de scherpte zelf veranderen. - You can change the brightness and the sharpness yourself.

The past participle of veranderen is veranderd. To use it as an adjective before a noun, you add the standard -e ending to get de veranderde helderheid. Because the participle ends in a -d, its spelling doesn't undergo the vowel or consonant shifts seen in verbs like vergroten (de vergrote foto), but it follows the exact same grammatical rule.

Both helderheid and scherpte come from adjectives. Is there a rule for using the -heid versus -te suffix?

Both suffixes turn adjectives into nouns, much like the English suffix '-ness'. The -te suffix is usually added to short, basic adjectives describing physical dimensions or sensations (like scherp becoming scherpte, or diep becoming diepte). The -heid suffix is much more common and is used broadly for abstract qualities (like helder becoming helderheid).

Can I say je kan instead of je kunt here?

Yes, both are correct. Je kunt is considered slightly more polished and is often preferred in written instructions, such as software settings menus. Je kan is extremely common in everyday speech. Just remember that if you invert the order to ask a question, the -t drops out: Kun je de helderheid zelf veranderen?