test-path cmdlet in PowerShell
PowerShell test-path helps you to check if the file path exists or not. It specifically determines if all elements of the path exist, and returns $True if all elements exist and $False if any are missing. 
Check if file exists in PowerShell
To check if file exists  using PowerShell test-path, you have to use the below script:
if(Test-path $FilePath -PathType leaf)
 {
    # if true do something
 }
else
{
    # if false do something
}
leaf checks if the $FilePath lead to a file
Check if file does not exist in PowerShell
To check if file doesn't exist  using PowerShell test-path, you have to use -not() function as below:
if(-not(Test-path $FilePath -PathType leaf))
 {
    # if the file doesn't exist do something
 }
else
{
    # if file exists do something
}
You can also use ! as an alternative to -not.
Overwrite a file if exists in PowerShell
In your case, you want to overwrite a file if exists in PowerShell, so you have to do the following:
$FilePath = "c:\debug.too.txt"
if(Test-path $FilePath -PathType leaf)
 {
    # delete a file
   Remove-Item $FilePath
 }
 # Add a new item
 New-Item -ItemType File -Path $FilePath