Donations are pulled from the database so it can't show any other value than the one that exists. Unless you triggered that overlay before making the change and did not refreshed the page, I don't see how it can show a different value.
Go to siropu_donations_widget_donate
template and look for:
<xf:button href="{{ link('donations/donate') }}" class="button--cta button--fullWidth siropuDonations-donateButton" overlay="true" fa="fas fa-donate">{{ phrase('siropu_donations_donate') }}</xf:button>
Replace this part: {{ link('donations/donate') }}
with your own link. Also, remove overlay="true"