Formal:
* With all due speed
* At your earliest convenience
* Immediately
* Expeditiously
* With the utmost urgency
* With promptness
Informal:
* Right away
* ASAP (abbreviation)
* Quickly
* In a jiffy
* Pronto
* First thing
More specific:
* By [date/time] (e.g., "by Friday")
* Within [timeframe] (e.g., "within 24 hours")
The best option will depend on the context and your desired tone.