Click on the pic avatar on the top right corner of the site, click on cashier, it'll open another section, then click on withdrawal.
A new section will open which will show your bank details, i e the bank details of the account you registered with.
Enter the amount you want to withdrawal then press withdrawal.
Note that, you cannot change the bank the withdrawal is going except if the name on your account tallies with the name of the new account you want to change to.
Note2 : Some banks doesn't do withdrawal with Bet9ja, in case you have difficulty withdrawing you'll need to change bank, if the condition cited above is met, otherwise you'll have to transfer the funds to another bet9ja account then withdraw from that account.