Here are three ways to express the same idea without his:

1. Revise to make the antecedent a plural noun.

All citizens should know their legal rights.

2. Revise the sentence altogether.

Everyone should have some knowledge of basic legal rights.

3. Use both masculine and feminine pronouns.

Everyone should know his or her legal rights.

This third option, using both masculine and feminine pronouns, can be awkward, especially when repeated several times in a passage.