By Default your Instagram credentials will be the same as your Facebook credentials. If you changed your Facebook password after creating your Instagram, maybe your password is still the old password. The best you can do is try until you get it right.
To get separate password you have to create new one for Instagram.
First login to your Instagram account as you login regular with Facebook.
Go to your Instagram profile and click on Edit Profile.
Enter the Email address you wish to use for login (if there is no email address associated with the account, and make sure you have access of this email), set a Username (if you have not set any, you can use your username instead of email id with password to login your account).
Now go to https://www.instagram.com and tap Forgot? next to Password on the log in screen to reset your password:
- To reset through your email address, tap Username or Email, enter your username or the email address you used to create your account and tap search. Choose Send a Password Reset Email.
- To reset through Facebook, tap Reset using Facebook. You may be asked to log into Facebook. You can then enter a new password for the Instagram account that was most recently linked to your Facebook account.
Note: If you can’t access the email you registered with and you didn’t link your Instagram account to Facebook, we’re not able to give you access to this account.
After resetting your password you can unlink your Facebook account from Instagram first and then you can connect it with other Facebook account or Page.