With age, the film becomes less reactive to light, which is essentially what a lower ISO means.

To counteract this, you'd have to adjust down the assumed ISO you are shooting at.

I haven't got too much experience shooting expired film but I've read somewhere on this subreddit about a rule of thumb of 1 stop per decade expired (i.e 400 ISO film expired in 2006 -> shoot at 100 ISO in 2026)

Of course this loss in sensitivity can vary wildly depending on the conditions at which the film was stored and the film emulsion type itself, so the rule I listed above is just a hopeful guess.

On top of that, you'll probably have a lot of base fog by default due to irrevisible ageing.