To reduce inflation, there are only two things you can do:
1. Wait for Infrastructure 5, then promote Mayors to Governors.
2. Hold your "minting" of money to an absolute minimum, so that you gain little to no inflation each year, then wait for random events that give you deflation (Exceptional Year, for example).
To avoid inflation in the first place,
1. Make certain you have promoted Bailiffs to Tax Collectors in all your provinces (accept inflation to do this),
2. Turn your military maintenance spending down to 50% when not at war,
3. Reduce your army size to the limit that can be supported without paying a penalty (click on the helmet to see this limit),
4. Further reduce your army size, if needed, to reduce yearly maintenance costs to the amount you get in census taxes each year,
5. Do not send monthly income to the Treasury unless absolutely necessary, or unless you use it to purchase something really important for your economy, like a manufactory, or troops with which to beat up another country, thus allowing you to take provinces, etc.