If you want to requalify out-of-date cylinders, you need a Requalifier Identification Number (RIN) or Visual Identification Number (VIN). These are assigned by the USDOT. Information for a RIN, is available on the USDOT website, which includes the application and procedures. Your retest facility will be inspected and your qualifications will be reviewed prior to DOT assigning a RIN. 

The qualifications for a VIN are slightly lower, but you are restricted to only a visual requalification. There are several options to apply for a VIN, visual only cylinder requalification, on the USDOT website. 

For those performing a requalification, there is a convenient log available for keeping records. Failure to keep good requalification records puts your RIN/VIN at risk of being cancelled by DOT.

USDOT has 1-page guidance document that provides a good overview of Cylinder Requalification.

You may search online for a place that has a RIN and offers cylinder requalification using the USDOT PHMSA locator site.