Yumna is a good and popular name for girls and it means God's blessings and bounty. The original meaning is "a woman who belongs with the people of Al-Yameen [the right side]" on the Day of Judgment (mentioned in Surat al-Waqiah 56:27):
27. Those on the Right, how fortunate are those on the Right!
28. They shall recline on high amidst lote trees without thorns
29. Among Talh trees with flowers (or fruits) piled one above another,-
30. and shade extended,
31. and flowing water,
32. and fruits in abundance,
33. never-failing and never out of reach.
34. And on Thrones (of Dignity), raised high.
Those who tell you to change the name are wrong to say that and they are confused because of another word mentioned in Surat al-Qiyamah (75:37). This too is "Yumna", but it is not a name but a verb, and it means "[semen] being ejaculated". This word is completely different from Yumna name and it comes from a different root [M-N-Y] while the Yumna name comes from [Y-M-N] root. So you can rest assured about the meaning of this name, and to be more sure you can look at this people search engine page which shows hundreds of women and girls named Yumna: