I know that your place in jannah will be decided by your good deeds but once you are in jannah can you advance to higher levels of jannah while in jannah? or your place in jannah can only be decided by what you do in dunya? Also does it mean that every bad deed you do, you are actually decreasing your level in jannah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The believers will enter Paradise by the Mercy of Allah. Out of His mercy, Allah made their deeds a cause of attaining that Mercy, but Paradise is not a recompense in return for their deeds; rather, the deeds are just a reason: the reason and its cause are achieved by the virtue of Allah.

The people of Paradise will take their positions by the Virtue of Allah on them.

His Wisdom required that they will take their positions in Paradise according to their deeds. However, Allah is the possessor of great bounty: He raises the degrees of whomever He wishes.

Therefore, if the mercy of Allah implies raising the degree of the slave in Paradise, then Allah raises his degree. The degree of people may be raised in Paradise due to intercession, as Allah raises the children and offspring to join their parents even if they did not do the same deeds as their parents. However, they are raised so as comfort the eyes of their parents by the presence of their children with them.

For more benefit, please refer to Fatwa 360298.

The matter is all in the Hands of Allah, and what His Wisdom implies.

A wise person should strive to do what benefits him, acting according to the obedience Allah, avoiding committing sins, hoping in the Favor of Allah and His vast Mercy.

Allah knows best.