Fixing A Mistake: Back in Beckman Coulter, Inc.

I realized I made a mistake yesterday in selling Beckman Coulter, Inc. (Ticker: BEC). So how do I fix it? I sold the shares of Mastercard Incorporated (Ticker: MA) that I bought yesterday @ $218.21 resulting in a break even trade. I added back my Beckman Coulter position @ $74.86, $75.09 and $75.10. I want to continue to build a good position in Beckman Coulter because I have a strong conviction on this one.