Odoo 15 is still in beta and based off the fact you’re asking this question, I suspect you’ve never been through a major version upgrade.

If you do not want to wait for the OCA OpenUpgrade, and do not want to pay Odoo to upgrade your database, you can prepare now by installing the master repo and then using PGAdmin or similar to compare the databases to find new columns and tables. After that, you can use a script to map the data to a new DB or use a tool like Navicat to help. You could start on community modules but you wont have access to enterprise.

Keep in mind the master branch is still beta and is subject to change. If  you're going to start anywhere then that is it.

It's substantially more work than it appears especially if you have quite a few records. I highly suggest you reach out to someone who has done this in the past or hire someone to be available.

Odoo SAAS upgrade is free but self hosted is $10 per line of code ($$$$$)