If you already entered income information, and you are trying to change the order of the names, you will not be able to do it successfully. Trying to change the names will cause numerous errors in your tax return that you will never be able to straighten out. You should just clear the return and start over from scratch.
If you filed a joint return last year with the same spouse, you have to keep the names in the same order that they were in on the previous return. The IRS instructions for Form 1040 (page 17) say "If you filed a joint return for 2017 and you are filing a joint return for 2018 with the same spouse, be sure to enter your names and SSNs in the same order as on your 2017 return."
