Green’s Dictionary of Slang

to smarten up.

1979		Arizona Daily Star 22 July J3/2: The man doing it was an interior decorator, not an art conservator, and he did what he felt was best – he went in and spiffed up the church [OED].
2021		J. Ellroy Widespread Panic 154: The [listening] post was spiffed, spangled and space age [...] the latest and greatest new stuff.