how can i show her ive changed?

[Barry]

Not sure what you lied about in the first place. probably covering up for something.

Well surely if you stopped lying about whatever it is she'll notice it anyhow without having to sign a proof document of honesty. She probably loves you just the same as always but she will not trust you very well for certain lies but there again it also depends on what you lied about & why.

Come what may do not show her a weakness because she will get into that weakness and widen it.
Instead just show her affection and merely be yourself.
As a PS on this i wonder if she always tells you the truth. Oh yes that honesty thing goes both ways.
I hope this helps you on the path. All best