There are few bugs in the code solve it and also make some changes in the Android and iPhone app

Hello Freelancers, I have already developed an Android app and an iPhone app. Backend is PHP and MySQL. There are few bugs in the code, I will tell you clearly in the chat. This is a bit complicated. This is a Non-profit app and I cannot afford much. Please bid as lowest as possible. Please message me only if you've read the description completely. Add * beside Hello to let me know that you have read the description clearly. Skills required are :- iPhone app development(Swift), Android app development, PHP, MySQL, Facebook app integration. First change to be made is, I have Facebook integration in the app and all the code related to it is already written. But I lost that facebook developer account credentials. So now I will give you another account details, connect this account with the app. After this, I will tell you the changes to be made. The changes are very basic and very easy(eg: change buttons, change font, add splash screen), but the bug in the code is: Let us take Twitter as example, if I create an account on twitter in an iPhone app, I can that id to login in the iPhone and Android, right? But here in this app, when I sign up with an account it is working fine, if I login with the same account in different phone then it is taking this account as new account. Like this, the accounts are duplicated. Please read the description carefully before bidding. I cannot afford much. I can post thanks and write about you on my website. It is a Non-profit app and it will help you a lot.