If your water bottle's bottom has bulged out due to being kept in a freezer, it is likely that the water inside expanded as it froze, causing pressure to build up and deform the bottle. Unfortunately, it is challenging to repair a bulged water bottle. However, you can try the following steps to potentially salvage it:
Remove the water bottle from the freezer: Allow it to thaw completely at room temperature. This might help reduce the pressure inside and potentially restore the shape to some extent.
Gently press and massage the bulged area: Once the bottle has thawed, try pressing and massaging the bulged area gently with your hands. Apply gradual pressure and circular motions to see if the plastic will regain its original shape.
Fill the bottle with hot water: Fill the bottle with hot (not boiling) water. The heat might soften the plastic and make it more pliable. Leave the hot water in the bottle for a few minutes before carefully pouring it out.
Use warm air: Another option is to use warm air to soften the plastic. You can use a hairdryer on low heat setting and direct it towards the bulged area. Keep the dryer a safe distance away to prevent overheating or melting the plastic.
Use a heat source: If the above methods do not yield any results, you can try using a heat source like warm water or a heat gun (with caution) to warm up the affected area. Again, be careful not to overheat the plastic.
Keep in mind that these methods may or may not work, depending on the extent of the damage and the type of plastic used in the water bottle. If the deformation is severe or the bottle shows signs of damage or leakage, it is best to replace it for safety reasons.