Replace lmer coefficients in R

From the post below,

Replace lm coefficients in [r]

I am also interested in changing the coefficients of a mixed model fitted with lmer. For e.g. in a a model of the form below:

mod <- lmer(y ~ x1 + x2 + x3 + (1|class/subjects), data=data)

How can I change the coefficients of x1 and x2 to another number (0 or 0.1 or 1 etc).