Geranium with Brown leaf edges.
Thank you for your question about your geranium. The most common problem that geraniums with brown leaf edges have is a lack of water. Most garden stores sell little probes that tell you whether the soil is dry or wet.
The yellowing of the leaf, between the healthy (green) and dead (brown) tissue is called chlorosis, or a lack of nitrogen. This pattern is also indicative of an iron/manganese toxicity, which happens when the pH of the soil is too low (acidic.) I suggest you get an inexpensive soil pH tester, to see if the pH of your growing medium is between 6.0 and 6.6 (water is 7.0, so this is just slightly acidic). If the pH is below 6.0, you can add just a little lime (calcium carbonate) to the top of your soil, and mix in a bit.
However, they can also develop leaf problems such as these if they are consistently watered with tap water that has been chlorinated. Here is a link to an article that explains the benefits of rainwater compared to 'treated' water. You might try watering with rainwater (not that we have some) or spring fed water.
In the meantime, cut off the leaves that are browning, and hope that the few weeks we have left in the summer will mean more healthy leaves.
Hope this is helpful. Good luck!