Change input type by element inspection, [duplicate]

In general this can not be avoided, the legal would be to use JS for validation (or the language you are using). This way you can avoid any changes you do not want (the user can do).

In addition to the validation in the client (JS), never forget to perform the validation also on the server (depends on the chosen language, eg Php). If the user attempts to circumvent some validation on the client, he will not be able to circumvent the server so easily.