Can I increase or decrease the block reward an year after launching my altcoin? 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.