I've got a Novation impulse MIDI controller; I need to link different channel selection to different buttons of my MIDI controller, in the way of I can change instruments while playing only pressing those buttons. How can I do this?
What you're looking for is called a "program change", which is essentially a MIDI signal that tells the destination to switch instruments. In FL Studio a program change will switch through channels in the Step Sequencer, which is what you're looking to do.
Not all MIDI controllers can send program changes, but luckily the Novation Impulse can. (Bottom of the page.)
It sounds like you want to trigger program changes with the Impulse's pads instead. Once again you're in luck, since Novation controllers come with the Automap software. Automap - as you'd guess - automatically maps controls based on what software is active. Most of the mappings can be overridden, however, including program change events to pads.
I won't go through Automap step by step (I don't own Novation hardware), but both of the manuals for the Impulse and Automap cover setup and use extensively. I'll point you in the right direction though:
Automap 4 User Guide: - Page 11 has information specific to your Impulse
Impulse User Guide: - Page 29 covers Automap setup