Been there. Remove the inside AC cover and look for 4 long bolts like this. Yours may look different but they are all about the same.
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Give each bolt one 360 degree turn right to tighten. Look at the gray gasket if you can see it (mine was gray) as you tighten to see if you can detect the gasket compressing. If you do not feel any resistance tighten them a little more but do not over tighten. If you can physically see the gasket compressing do not tighten any more.
Wait for rain or use a water hose to wet well around the roof AC unit. If it still leaks turn them 360 degrees again. If this again fails replace your seal.
Mine failed after 6 years and I had to replace the seal. Easy two person job.