Use the -replace operator[1] to perform (invariably global) string replacements (since it is regex-based, a verbatim \ must be escaped as \\); the automatic $PWD variable contains the PowerShell session's current location (which, if the underlying provider is the FileSystem provider, is a directory):
$PWD -replace '\\', '/'
If you want to ensure that the resulting path is a file-system-native path (one not based on PowerShell-only drives):
$PWD.ProviderPath -replace '\\', '/'
If there's a chance that the current location is from a provider other than the file-system (e.g., a registry-based drive such as HKLM:).
(Get-Location -PSProvider FileSystem).ProviderPath -replace '\\', '/'
[1] In this simple case, calling the [string] type's .Replace() method is an alternative, but the -replace operator is more PowerShell-idiomatic and offers superior functionality. This answer contrasts the two.