Unless you were required to submit evidence of insurability (EOI), your Summer Enrollment changes begin on September 1. Your PY21 monthly premium and TexFlex deductions start with your October 1 paycheck.
Coverage that requires EOI approval begins the first of the month after the approval date (if you did not get approval before September 1).
Plan Year 2021 Rates and Comparison Charts
2020 Summer Enrollment materials (for information only)