You can adjust the ticks and ticklabels of colorbars in (at least) two ways. At construction:
xs = ys = range(-1, 1, length = 101)
Z = xs.^2 .+ ((ys)').^2
fig = Figure()
ax = Axis(fig[1,1])
hm = heatmap!(ax, xs, ys, Z)
cbar = Colorbar(fig[1,2], hm; ticks = ([0.5,1.0,1.5], ["Low","Medium","High"]))
fig
or after construction:
cbar.ticks[] = ([0.5,1.0,1.5], ["0.5","1.0","1.5"])
I could not fully understand what problem you are trying to solve specifically, please provide an MWE (Please read: make it easier to help you) if the above is not sufficient to solve your problem.