Hi @TarrahArshad,
The Crowdsales documentation can be found here: https://docs.openzeppelin.com/contracts/2.x/crowdsales
You could take the ideas from IncreasingPriceCrowdsale, extend Crowdsale and in a getCurrentRate function specify the rate after specific times.
I put together an example of manually setting a crowdsale rate using this. 
This code has not been tested or audited. Please appropriately test and audit before using in production.
Setting crowdsale rate manually
If you are selling based on a set fiat value, then you may be better to create a Crowdsale that accepts a stable coin rather than Ether. You could use the ideas from OpenZeppelin Contracts to do this.
Please see the documentation for calculating rate: https://docs.openzeppelin.com/contracts/2.x/crowdsales#crowdsale-rate
I recommend starting with something simple for testing purposes, such as a rate of 1 (assuming decimals of 18).