Assuming that you have a complex baseband signal (i.e., IQ samples), then you need to multiply that signal by a $cos{(2{\pi}ft/Fs)}$

In python this would look something like this:

N  = 1000          # samples in input array
Ts = 1.0 / 250e6   # sample period
f  = 1000e6        # output frequency
t  = np.arange(0, Ts*N, Ts)
y  = x*np.exp(-1j*2*np.pi*f*t)

Where x is an input array of baseband I/Q samples and y is the tuned output.

Assuming that you have a complex baseband signal (i.e., IQ samples), then you need to multiply that signal by a $cos{(2{\pi}ft/Fs)}$

In python this would look something like this:

N  = 1000          # samples in input array
Ts = 1.0 / 250e6   # sample period
f  = 1000e6        # output frequency
t  = np.arange(0, Ts*N, Ts)
y  = x*np.exp(-1j*2*np.pi*f*t)

Where x is an input array of baseband I/Q samples and y is the tuned output.

Keep in mind that if your signal is not sufficiently upsampled, then you won't be able to modulate to the desired output frequency from your question.