Open the Messages app
Delete unwanted conversations
Open Settings
Tap General
Tap iPhone Storage
Tap Messages
Review and delete large attachments
Delete old photos, videos, and files in Messages
Turn on Messages in iCloud if you want messages to sync across devices
Go to Settings
Tap your name
Tap iCloud
Tap Messages
Toggle Messages off and choose to disable and delete from iPhone if prompted
Restart your iPhone
Update iOS if needed