Assuming that the iPad is not centrally managed (or owned) by your former employer, signing-out from the currently signed-in AppleID (now known as your Apple Account) should be straightforward. If instead the iPad is centrally managed by your former employer, you will require assistance from the Administrator responsible for managing the device.
To sign-out from the current AppleID:
Settings > [Your Name / AppleID] > Sign Out
During sign-out you will be presented with the option to delete or keep a copy of associated iCloud data. After you have successfully signed-out, you can simply sign-in again with your private AppleID credentials.
In more detail...
Change AppleID Account
If you do wish to change and use an entirely different AppleID, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My service from iPad/iPhone settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
Having disabled the Activation lock, you can now sign-out from your old AppleID:
Settings > [Your Name / AppleID] > Sign Out
Presumably you’ll want to keep any iCloud data that is locally stored on your iPad. If so, accept the opportunity to keep a local copy of your data (e.g., Calendar, Contacts, Notes, Keychain Passwords etc.) - as otherwise this data will be wiped from your iPad as you sign-out from iCloud.
Also ensure that you have signed-out from Messages and FaceTime:
- Settings > Messages > Send and Receive > [AppleID] - scroll to the bottom where you will find your signed-in AppleID highlighted in blue text; tap and select Sign Out
- Settings > FaceTime > [AppleID] - again, scroll down the page to find your signed-in AppleID highlighted in blue text; tap and select Sign Out
After signing out, a restart of your iPad/iPhone is recommended:
Restart your iPad - Apple Support
Force restart iPhone - Apple Support
After restarting, you can sign-in to your new AppleID account from iPad settings - and also sign-in to Messages and FaceTime if necessary. Any iCloud data that you saved to your device during the previous sign-out will be merged with your new AppleID account.