Can I increase or decrease the block rewards or coin per block once I make my coin public and people start mining it.
You can do whatever you want, so long as the people who mine it agree to mine on top of your change.
In Bitcoin, the block reward goes down on a set schedule. Everyone knows this, and it's baked into the protocol. Any mining nodes producing blocks that don't conform to this (or any other part of the protocol) are wasting their effort since nobody will accept their blocks. The rest of the network will continue as if nothing happened.
You can create a coin that says "on Sep 18 2018, the block reward will half", and have that rule baked into the protocol. Maybe people will mine it...maybe they won't. If they do, they do so knowing that the block reward will change on that day, and will reject blocks from nodes that don't follow that rule.
You could also create a coin that says "the block reward will never change", and then have some mining nodes change it anyways a year from now. This would create a hard fork, and again, maybe people will mine on top of the fork...maybe they won't.
If you know in advance how you want it to change, then you can include this as part of the rules for calculating the correct block reward. The most common way is to base it on block height rather than date, and estimate (based on your target block time) how many blocks will be mined in a year. For instance, "if block height < 10000 then reward = 10 else reward = 5". This is how Bitcoin implements its decreasing block reward schedule.
If you change your mind later, you could release a patch which changes the reward. This will result in a hard fork of your coin between those who adopt the patch and those who do not. You will have to hope that your argument for a changed reward is sufficiently convincing to your community that the vast majority of users choose to adopt your patch.