Hello Fupayme84,
For help with the iMessage app, you can only go back as far as your have previously set your message history for. While there's no guide for the iPhone the steps for checking and changing your settings are similar to the ones for the Mac, as in the guide page found here: Change General settings in Messages on Mac
On your iPhone you can navigate to Settings > Messages > Message History and view your current settings for how long your messages are kept. As mentioned in the article above "Choose how long to keep message conversations in your account. For example, if you choose 30 Days, your conversations (and all attachments) are available in Messages for 30 days. After that, they are automatically removed"
Additionally, if you're able to view the content you can take a screenshot of the conversation you need to then print it out, or search the App Store for a comparable app for your needs. Get apps in the App Store on iPhone
Take a screenshot on your iPhone - Apple Support
You can also use those steps to check other devices you may use with iMessage. If you need further assistance or a full history, you'll want to consult with your legal counsel to further assist.
Cheers.