I created a new sandbox but since its a copy from production it is on API 44 all other sandboxes are API 45. is there a way to manually update the API version on the newly created sandbox to API 45?
please read dedicated article about it Sandbox Preview Instructions for the Salesforce Spring '19 Release.
you can't update sandbox API version manually. You have to wait for Production upgrade and after that do one more sandbox refreshing.
to check date when upgrading is planned use status site. For example, for NA89 instance Spring'19 release is planned on Feb 09