start with this - https://support.apple.com/en-us/HT204306
if that doesn't work and if you absolutely cannot remember your 6-digit passcode, there's the nuclear option. download and install 3uTools on your PC, boot up the phone and connect it to your PC, use the Flash feature and do an anti-recovery flash.
WARNING - BE AWARE!!!!!!!!
- this will update you to the latest IOS and return the phone to OOTB condition and you will have to set up your phone again
- all your data will be lost, i hope you have a recent backup.
- if you do have an iCloud account connected to the phone it will still be there, hopefully you can remember that password - however, if you can't there are methods via appleid.apple.com or www.icloud.com to recover/reset that password, provided you have access to that account's email and the phone number for two-factor authentication is still good.
Hope this helps. Good luck
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Have you bought the phone second hand or have you had it from new?