Astable timer: Halve frequency while maintaining the same "up" pulse width?
The charge period is determined by R1 and R2 charging the capacitor. The discharge period determined by just R2 and the capacitor. Given that you can halve the frequency by doubling the capacitor you now have the relationships to recalculate the resistors remembering that if the capacitor doubles, R1 plus R2 has to halve to get the same charge period.

Alternatively go to one of the many 555 website calculators and try things out online.