Amending rent due from 12th to 1st of month in middle of contract

[Nick Gagel]

I currently have my rent due on the 12th of each month. Mortgage is due on the first. I now know it is better to have that date be the 1st of each month. 

2- What is the best way to go about doing this without losing out on money? Or is a refund of those 11 days and amending to the 1st the only way to go about it? This is a 2 year contract and thinking of amending it at the end of this month which is the end of the first year. 

Or, is keeping it the 12th of each month for the rest of the year best?  

Thank you!