Most mispronunciations are not something you can correct by telling the child they are saying it wrongly. One of my children had a persistent use of the
n sound where
l belongs (eg eating nunch) and I kept asking my doctor who kept saying
n is one of the last sounds for children to get right. I kept countering that if my child was saying "wunch" like the other agemates I would calm down, but this was an entirely different thing. I was reasonably worked up about it, and then at whatever late age they are supposed to get
l right, the problem disappeared.
Lisps are super common in toddlers - that's why people imitating baby talk use them, along with "I wuv woo mommy" and the like. Relax, understand her, cause her to believe she can get her thoughts across to you, and don't worry that it needs correcting until you get to the age where it is supposed to be correct. By then it almost certainly will be, and if it is not then she will be old enough to understand whatever you need to explain to her as part of correcting it.