I can't change my screen time passcode

See the Apple Support article(s) below. 

Screen Time passcode reset

If necessary,  you will need to restore the device to factory settings. Do not restore from a backup made when the Screen Time code was active.

Restore to Factory setting.      

Have you searched for the App Store? If necessary, do a factory reset using the link above.

Search on your iPhone, iPad, or iPod touch