Howto change current user password

You can remotely set the registry key

[HKEY_LOCAL_MACHINE\SOFTWARE\RealVNC\WinVNC4] "Password"=hex:xx xx xx xx

to figure out what hex value to set you can set it on a test machine and copy the resultant value, or you could use a tool which there a many of to generate one for you

EDIT

it even looks like there is an app for this I havent tested this I have only just searched for it on google, but there are a load of results with similar promises

(Edited per comments):

As noted, RDP should generally not be directly exposed on the public Internet. Limiting this exposure can be done in several ways, through simply blocking port 3389 access except over VPN, to using RD Gateway for a more advanced solution. If you have an IPS or IDS+Firewall that supports it you can use them to block hosts with repeated login failures.

For internal brute-force protection you can set lockout policies in the Local Security Policy. There are settings for account lockout duration, account lockout threshold, and how long to wait before resetting a lockout.

You can use secpol.msc to modify these settings: secpol.msc -> Security Settings -> Account Policies -> Account Lockout Policy.