HEPES buffered saline solution contains salt or salts, hence "saline", in addition to HEPES, so it is not the right starting material for making a solution that is just HEPES.

If you have access to a pH meter and have solid HEPES, you can make the desired solution.

For example, starting from solid HEPES, which is acidic because it is a sulfonic acid, you will adjust the pH upwards using a solution of concentrated NaOH (using personal protective equipment). For 1 liter of 25 mM HEPES, prepare a solution that has the right amount of solid HEPES (5.96 g) in 900 ml of distilled water. Start measuring the pH with a calibrated pH electrode, with magnetic stirring. Add 1 M NaOH dropwise until the pH reaches 7.4. Bring the volume up to 1 liter with distilled water.

You can also purchase HEPES buffer solution at 1 M from Sigma Catalog number 83264 or H3537), which has not had its pH adjusted. You could dilute that to a little over 25 mM, add NaOH to adjust the pH, then bring the volume up to bring the concentration to 25 mM.

You can also buy HEPES buffer solution from various suppliers with the pH adjusted to 7.4 or 7.5 already, although it will be more concentrated than 25 mM. The pH may change a little when it is diluted to 25 mM, so you may want to adjust it with either NaOH or HCl if you require the pH to be exactly 7.4.