The order in which the buttons for different identity providers are shown in Azure AD B2C is based on the order in which they are saved. If you select multiple identity providers and then save, they will be added in a seemingly random order. However, if you add one (say Facebook), save your changes to the policy, and then add another (say Google) and save. They will be displayed in that order. This also applies for the Local Accounts button in the policies that show such button.
In the case of Sign-up/Sign-in, the section for local accounts will always be displayed at the bottom and there is currently no way to control this via Azure AD B2C.
You can leverage hacky CSS to do flip these two sections, but beware that this is a hack and can break if at any time the html elements change.
div#api { display: table }
.social { display: table-footer-group }
.localaccount { display: table-header-group }
You should probably request this as a first class feature via the Azure AD B2C UserVoice forum