How do I get lilacs to re-bloom?

All depends on what variety of lilac you have. Some only bloom once in spring.

https://homeguides.sfgate.com/many-times-lilac-bush-bloom-61177.html

The standard old fashioned (French) lilacs only bloom one time a year, mid spring- depending on location. The dried up blooms can be snapped off to tidy up the plant. Break at the base of the bloom or they will not re-bloom for several years on that spot. Some newer varieties are called re-blooming lilacs. The I've seen don't looks the same

They only bloom once a year in the Spring. Grow in full sun,provide lots of water. Most lilacs don't require pruning until they reach about six to eight feet tall. The best time for pruning lilac bushes is right after their flowering has ceased. This allows new shoots plenty of time to develop the next season of blooms. Pruning lilacs too late can kill young developing buds,so do not prune off tips that is what they bloom from the following year unless you do it right after flowers die. JUst have to sit back and wait for them to be glorious each Spring. If you cannot be without them/the to die for scent get yourself a Lilac candle from Yankee candle company. I live in desert now and buy these candles because Lilacs will not grow here. Gives me that beautiful one of a kind scent I miss from when I lived in Midwest. I have actually been burning this exact candle the past few days. I love the actual flowers but this candle is too die for.......

I have a miniature Korean lilac bush that flowers like crazy in early June. The entire bush is covered in lilacs for about 2 weeks and smells heavenly! Also covered with a few different varieties of butterflies and bees! It will rebloom in August but I only get a few small blooms-less than ten. Don't expect much for a second bloom. If you just planted your bush this year and it was in bloom when you planted it you probably aren't going to see any more blooms until next year.