how to export text messages

If you have a Mac computer and have turned on Continuity for messages you can do it from the messages app on your Mac.

If not and you do have a Mac you can acquire the third party software Phoneview. If you have a PC try TouchCopy.

Check iMazing, has versions for both Mac and Win. Accurate layout with all timestamps/media/contact name + actual phone number if needed for legal case (guide if needed. Ignore that it's for WhatsApp, steps and options are the same). Haven't seen any free app with this option, idk about Continuity feature though mentioned in another comment.

iOS offers no way to do that.

Best suggestion I can offer, contact your cellular carrier.

They should have your text history.

iMessages won't be included.

Good to know about continuity.

Glad to hear it's sorted! Regarding iMazing, I also like that they have search by date range, quite handy sometimes. And PDF export formatting is quite flexible too.