Hi,
You have to change/upgrade your subscription to use odoo sh, before that you have to ensure that you are having odoo 17 or odoo 16, if you are using versions like 17.1, 17.2 etc, you cannot move to odoo sh.

If you are using odoo online versions, you have to wait untill the release of 18 and once the online db is upgraded to 18, you can think of moving to 18.0 in odoo sh

Thanks