You will need to replace a leaking bathtub drain flange as soon as you discover it, otherwise you risk having water damage to the floor in your bathroom.
Tip # 1 - Use the Right Tools
To replace this, the drain basket needs to be removed. The only thing you need for this is a basket wrench, which is a specialized tool available in hardware stores that perfectly fits the drain basket and will lift it out without damaging it. If you try to remove the drain basket with any other tool you risk damaging the basket. The most common issue that comes up due to improper tool use is that the cross at the bottom of the drain basket breaks. The basket wrench is the only way you can prevent this from happening.
Tip # 2 - Clean the Bathtub and Drain Hole
Cleaning the drain hole before you start installing the drain basket is very important. This is because when the previous installation was made, a lot of putty was used to hold the drain basket in place. If you try and install it without removing this dried up putty you will not be able to create an effective seal. It is important to clean the area before you start installing a new basket.