Plan your timing
If it's going to be hot, you might want to avoid training in the midday heat. If that's not possible, just remember that you heat up when running, and this will happen quicker if it's hot outside. To combat this, you'll need to slow down to make sure you can complete any training sessions. Expect to see your paces down a bit when running in the heat.
You may end up sweating more in the summer. You'll need to drink more to combat this. However, it's possible that just water won't do it. When we sweat we lose electrolytes too, and if you just drink water you're going to dilute the ratio of electrolytes to liquid even further. If you are a very salty sweater (have lots of white stuff over your face after a run!) think about popping an electrolyte tab into your water.
Think about clothing
In general you're going to be wearing less clothing than the winter, make sure you're using modern fabrics which will wick away moisture from the body.
You might want to wear a hat or sunglasses to shield your eyes to help keep your face relaxed. If you are using a cap, make sure it's well ventilated and not trapping heat in! Often I'll pour some cold water onto my hat to help cool me down
You may need to wear some sunscreen too if it's really hot. The issue here is that when we sweat the sunscreen can start to sweat out too, and if anyone's ever had it hit their eyes, it isn't enjoyable! Having a sport focused sunscreen can really help.
Try going off-road
The summer can also be a great opportunity to explore some off road routes. The surface will generally be lovely to run on, you can get onto some forest trails and woodlands, where the shade of the trees will offer some respite from the sun. There's some wonderful countryside to explore.