In everyday English:
- If I were her/she, I wouldn't go there.
- If I were in her shoes, I wouldn't go there.
Similarly: what would you do if you were me/I?
Sticklers for accuracy (puntičkáři), especially AmE speakers, may tell you that “if I were she” is technically correct, but most speakers wouldn't use it, even in a formal context.
(It's the disjunctive pronoun.)
- Věřili byste, že je to ona? Would you believe it's her?