There are a variety of environmental factors that can affect your tefillin. Heat can cause some of the lettering to melt with it housing to warp. Cold, can cause the lettering to crack. moisture can cause some of the same issues.
The closer you get to the extremes for any of these environmental factors, the more likely you are to incur damage.
I wish I had more specific instructions to give you, but currently I don’t because there are too many variables, such as how old they are, the quality of the hides they were made from, etc.
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