Is it possible to increase bounty?
Like I have set bounty to this question to 100, but I would like to raise it to 150. Does the system allow me to do so?
You can't change it by yourself. You can either wait for the bounty to end and start a new one.
Another option is to flag the post for moderator attention and mention about wanting to increase the bounty. Mods can then refund the existing bounty and you can offer a new one.
Be aware that with this, a new bounty is created and as a result, if someone had originally posted an answer after the bounty was issued the first time, once the second bounty has run it's course and you didn't allocate the bounty, the person who posted the answer may not be automatically awarded the bounty rep.