For example,the default is "yytext",but I want to rename it to "aatext",how can I do this?
It seems like the Flex manual would be the natural place to look. Google offers several versions of Lexical Analysis with Flex, for various versions of Flex. For example, https://westes.github.io/flex/manual/.
The manual has a nice table of contents, which lists a chapter "Scanner options", which sounds promising, and a section within named "Code-Level and API Options", which seems like it would be just the thing.
And Lo! In that section we can find documented the prefix option, which can be spelled -PPREFIX or --prefix=PREFIX on the Flex command line, or %option prefix="PREFIX" in the input file, and which
changes the default 'yy' prefix used by flex for all globally-visible variable and function names to instead be 'PREFIX'.
Do read the rest of the documentation for that option, too.