Help please! PEM file

hello!!!, well, I am trying to update my app in google play, but it asks me the following:

"Your Android App Bundle is signed with the wrong key. Make sure it is signed with the proper signing key and try again. The App Bundle is expected to be signed with the certificate containing the fingerprint
SHA1: 5B:BA:CB:0E:25:FD:BD:C0:3C:47:B3:0B:72:F0:91:C8:0D:C5:C3:15
, but the one used to sign the App Bundle you uploaded contains the fingerprint
SHA1: 20:5F:39:2C:1D:77:87:5C:8E:27:12:A5:C2:29:2F:05:3F:98:D2:A5"

from google support they ask me for the PEM file to be able to restart my app ... can someone help me to get that file? I'm really lost Thanks!

They just send me this link but I can't solve it.