The best way to fix that is a new cap. You can caulk or repair it, bit it won't be a permanent fix, it will be a band aid.

Chip away the old cap. Those brick look like they are solid, so you might break some of them loose, if they become loose, you need to remove the old mortar and re-lay them.

Use mason sand and Portland cement to make the cap concrete. Mix it stiff. Use 2.5 parts sand to one part Portland. Put it on thick. Trowel the edges as straight up as you can, don't feather the edge like in the pic, the edge should be 1.5-2" tall, then thicker from there. The more concrete on top the better. If it's warm outside, cover it with plastic sheeting or wet blankets or towels for the rest of the day, after it sets for 30-60 minutes. Use a steel mason or concrete trowel to finish the top.

Use type S premix to fill any voids in the mortar. Mix it fairly thick.

Mortar and concrete loses strength when it's mixed too wet.

 

I'll add that mortar work is not that tough, but you'll probably want to do a practice run on the ground. I re-did my mailbox and it came out quite clean for a first time job.

 

ok will do! yea in SoCal so it's really hot dont want to be up there messing around for too long

 

You could wait until the fall when it's cooler, then take your time. I used the design described in the article below when I replaced ours. The overhang and drip edge keeps most of the rain off the bricks, which will help their longevity. And there is no thin layer of easily broken concrete/mortar at the edges, like in your photo.

https://www.jlconline.com/how-to/foundations/replacing-a-leaking-chimney-cap_o

 

yea i will for sure have to wait till it's cooler! thanks for the link!

 

If this is high with a steep pitch to the roof, respect it. Have a safety rope around your chest and under the armpits (in case you fall you don't want it close to your neck). Those brick definitely need tuckpointing, judging from the top photo, but that may be just the shadow cast from the sun in the photo. However, the lower photo appears not too bad, just fill in the small holes-no need to knock out all the concrete. If you tap on the cap with a hammer, does it have a "hollow-wood" sound and pieces easily break off, or a solid "whack" sound and nothing cracks. Hollow sound indicates it is loosening the adherence to the top bricks underneath. The "whack" means it is still tight and strong.
If that is the worst that the cap is in the picture, only one deep crack, I suggest just using an exterior caulk to fill in. Otherwise, it's not pretty but seems pretty solid to me. GE All Purpose 100% Silicon caulk is a good one I have used for my chimney.
After, you definitely want to apply a good, clear seal to help protect against weather. There are several good ones on the market.

 

About 20 years ago my chimney cap was cracked like OP's. A buddy and I replaced it with a precast cap. Worked out well and still looking good today. Something like the one in the attached pic.

 

Oh damn that would save some time where you find

 

Actually, this picture was from a link from Walmart. I didn't buy mine there but I don't remember where I got it from. The one in the pic cost $179 or something like that. We just set mine in a bed of mortar. Pretty simple.