Turns out the option is hidden. You need to go into settings and then controller. Once in the controller tab you need to scroll down completely and then click next to the modify button. Yes! in the black space!! Find screenshot below. Move your cursor in the same position as mine and then tap X.

Switch “Custom Look Controls” to On. Now you can adjust your aim assist settings!

Update: As from Season 9 you should see the Advanced Look Controls… button!

If this was helpful you can also check how to enable bots in firing range at: https://everythingtech.dev/2021/07/how-to-enable-bots-in-firing-range-in-apex-legends/ The perfect way to warm up!

You would also be interested in the best aim assist settings at https://everythingtech.dev/2021/07/best-apex-legends-aim-assist-settings/