I just want to know those laptop like Latitude 7300 come with DDR4-2666 memory, and operate at 2400 only. Can we replace it with DDR4-2400 memory? or have to use DDR4-2666? Thank you.

yes you can use and its work but if processor support 2400 MHz so its used only 2400 Mhz either your ram is 2666 Mhz .

My suggestion you don't purchase 2666 MHz if it not support , you waste your money .

Dell say : 2400 Mhz manufacture is stopped or not available but they provide in various dell laptop 2400 Mhz

“I asked about inspiron laptop latest modelsw which show dell online with specification of RAM is 2666 MHz but its Intel processor model not support 2666 MHz its support 2400 MHz on Various models Like ( i3 8145U, i5 8265U) After this i research on it and in inspiron also have same issues like my laptop Bios Show 2400 MHz and OS Show 2666 MHz“

I have Inspiron 5481 i3, with 2666MHz memory but the Bios says 2400 MHz and the operating system (windows 10) says 2400 MHz. So I don’t understand why they say use 2666 MHz when the cpu speed is max 2.3 GHz.

2666 is the maximum RAM speed;  the other number is the base CPU speed.  Two different things.

There are CPUs that run the RAM at 2666, and there are faster CPUs than the one you chose.  And the speed of the RAM is determined by the CPU, since the CPU has the memory controller aboard.