To turn off your activity status on Instagram, follow these steps:

1. Open the Instagram app and go to your profile by tapping your profile picture in the bottom right corner.
2. Tap the three horizontal lines (menu) in the top right corner of your profile page.
3. Go to Settings.
4. In Settings, search for or navigate to "Messages and story replies" or look for the option "Show activity status."
5. Tap on "Show activity status."
6. Toggle the switch to turn off your activity status. When off, others will not be able to see when you are active or when you were last active on Instagram.

This change takes effect immediately, and if your activity status is off, you also won't see others' activity status. Note that turning off your activity status hides your online presence universally to all users, not selectively.