Low tech solution if it is a battery powered flash - turn it on with fresh batteries, and leave it on at first without discharging the flash immediately.

I would suggest 20 minutes on standby, pop the flash once on manula, reduced power if there is a way at first, then shut it off and repeat this for a few days.

This is a low tech way to reform electrolytic capacitors.

I have a poweredful older line powerd studio flash, a Speedotron 2401.
With it you can actually hear little arcs and snaps from time to time for the first 20 minutes of conditioning, if the flash has not been used for a while.

I am not super diligent, but I do try to condition the studio flash pack every three months if it has been otherwise idle.
The power packed into the big caps in thia unit can leave a a lot of havoc behind when they blow. I have seen some fried examples at the repair depot over time.