Hello and welcome to iMore! You a can choose to keep your texts/messages for forever, for 1 year or for 30 days. To choose which one, go to Settings>Messages, then scroll down to Message History and tap on Keep Messages and then select which option you prefer. Another thing to think about is to delete attachments, like photos or graphics. They take up a lot of space.
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here. iMore is a great place, full of helpful people. I hope you'll like it here.