No.
Giving something up for Lent is a purely voluntary act of penance, and it is not sinful to change one’s mind on it.
It would only be sinful to break the Church mandated rules of fasting and abstinence, i.e. no meat on Fridays, and fast on Ash Wednesday and Good Friday. The nature of the sin would depend upon whether it was done intentionally. If one actually forgot it was Friday, and had meat, that would not be sinful at all.
God Bless