How do you change the difficulty of the bots?

With the standard configuration, Counterstrike: Global Offensive will populate the teams up to a maximum per team with bots to keep the teams balanced.

These bots are a quite easy, how can I make them more challenging?

Solution 1:

Bot difficulty can be changed at the console and in the configuration files by changing the bot_difficulty variable, which can be set to one of the following values:

Easy:   0   bot_difficulty 0 
Normal: 1   bot_difficulty 1
Hard:   2   bot_difficulty 2
Expert: 3   bot_difficulty 3

There are also the following two commands which are supposed to re-scale bot difficulty depending on the average scores of the "human" players on the server;

bot_autodifficulty_threshold_low    defaults to -2
bot_autodifficulty_threshold_high   defaults to 0

bot_autodifficulty_threshold_low is the value used to determine that the bot needs to make itself harder, and bot_autodifficulty_threshold_high is the value used to determine that the bot needs to make itself easier. These values can be set to any number between 20.0 and -20.0 but apparently this may be bugged.