What does the e in the verb pensar change to in four of the six present tense conjugations?

The first thing you should keep in mind is that we have the following verb:

Pensar

Then, we must have in mind that the conjugation time is present tense.

Then, we must have in mind, that we are going to conjugate in Spanish of Spain. It is not the same, for example, if we conjugate in Spanish of Venezuela.

For the Spanish of Spain we have:

Yo pienso

Tú piensas

ÉL/Ella/Usted Piensa

Nosotros pensamos

Vosotros pensais

Ellos/Ellas/Ustedes piensan

We note that the change of the letter "e" is for "ie"

"e" is for "ie"