Get yourself a packet of champagne yeast. You'll also need fresh caps.
Rehydrate said yeast in water (or use liquid) in a sanitized container, get a children's medicine dropper with ML gradients like this,
Sanitize it, open beer bottle, suck up slurry into sanitized eyedropper, squirt 1 ml of yeast into bottle, then re-cap. Give the bottles a shake, and then walk away for another 2-3 weeks to give that yeast a chance to do the job.