You can't. Keep records of your roll results per die instead, and determine how severe the problem is
There isn't a whole lot you can do prospectively-- if you're pretty sure that the dice are not fair, you will have a hard time using them in a way that gets the same results as fair dice would. That is fundamentally what it means for a die to be biased. Some methods might compensate to varying degrees for specific manufacturing problems, but it strikes me as unlikely that you will find a method that exactly offsets your die's favor for 6s and 9s.
I can think of a couple of ad hoc things you could do to lessen systematic favoring of outcomes due to the bias, like having other players roll your dice in a rotation or shaking the dice in a cup before throwing rather than your hand. But these don't eliminate the problem; they just introduce more variation on other factors which could influence rolls (especially at the margins).
Something like a dice tower could help, but it won't necessarily cut out the bias and, crucially, it will introduce new systematic factors with an unknown influence on your roll results. It's possible, if not too likely, that the specific issue with your dice would be exacerbated by the tower, or might interact with it in some other systematic way that cuts against randomness. Inhibiting the rolling action, such as by rolling onto a towel, is likely to result in unworkable outcomes (like cocked dice) and may not counteract the die's issue very well, while also introducing new systematic factors that affect rolling.
What you can easily do is keep records of roll results to determine, after the fact, how severely your dice really are biased in practice. It's very easy to mis-estimate something like minor manufacturing issues with dice without empirical observation, and it can take a lot of rolls of a d20 to confidently assemble its distribution of roll outcomes. If the problem turns out to be pretty minor, it might not matter to you or your table. If it's severe enough to be an issue, at least you'll know and can stop using those dice.
Every other solution I can think of which would directly address your dice set's departure from fairness essentially cut the dice out altogether. The randomness a die introduces is based on its ability to physically move with a roughly equal chance of any face showing when it comes to rest. If that property is the one you don't trust, then you can't trust the dice.