I pad & Square app

I am not tech savvy . I use an I pad on a register stand for the SQUARE APP . Someone else originally set up this square app and I pad for me and used thier sign in I. D . However they are now unreachable and I dont have that info .

I simply tried to update the square app on that I Pad but I needed to sign into Apple on that i pad and it kept telling me my user I D or password were incorrect but I am certain that i was correct . sometimes the former persons user ID populated the square and it wanted his I D to sign in which I do not have as mentioned . (I own all the devices here )

So i would try my Apple sign in info again but now Im locked out and cant get it to let me in no matter what i do . I even changed my password.

Am i not able to sign into that device to update SQUARE because it was originally linked to him ? Keep in mind i was able to sign into apple with my info on this device that bIm using now so I have the correct info to sign in. I am also able to sign into my SQUARE account to operate the register with MY info as always - but on that I Pad I cannot sign into Apple to update the SQUARE APP . Im extremely frustrated . Please help