Formal:
* Si seulement je pouvais remonter le temps. (This is the most direct translation and emphasizes the wish.)
* J'aimerais pouvoir revenir en arrière. (This is more about wanting to go back in time.)
Informal:
* Si j'avais pu revenir en arrière... (This emphasizes the regret and implies that it's no longer possible.)
* J'aurais aimé pouvoir revenir en arrière. (This is similar to the previous one but focuses more on the desire.)
Other options:
* J'aimerais pouvoir changer le passé. (This focuses on wanting to change the past.)
* J'aurais donné tout pour revenir en arrière. (This highlights the strong desire to go back in time.)
The best translation will depend on the specific context and the desired tone.