You can subtract the intercept from your dependent variable and then simply run the regression as usual, but without the intercept.
More mathematically, let your model be $$y_i = c + \beta x_i'+u_i,$$,where $u_i$ is the error term and $\beta$ are the coefficient parameters. We can estimate this model for fixed $c$ by letting $y_i^*=y_i-c$ for all $i$. We can then estimate $y_i^*=\beta x_i'+u_i$ as usual.
If you want to check whether this is a good idea, I would recommend t-testing whether indeed $H_0: c=c_0$, where $c_0$ is the fixed value of $c$ you have in mind.
Do you know how to perform a t-test for the intercept?