I can not update my mac

There have not been any New Updates for High Sierra since November 2020.

IF you mean upGRADE to newer macOS - That is governed by the Age of the computer. About this Mac from Desktop will provide that required information.

macOS Mojave - Technical Specifications

macOS Catalina is compatible with these computers

How to get old versions of macOS

You will need to verify which version of macOS it qualifies to upGRADE to.

If downloading from next pages, us Safari as Others may not function. Also, make a Time Machine Backup before attempting.

Special Note : Catalina Does Not Support any Application, extensions or Drivers for 32 bit. Everything Must be 64 Bit or it will not work and there is not workaround.

What computer would that be?