Presumably you have a default route pointing out the leased line interface. The easiest way to use the DSL interface as a backup is to configure a so-called 'floating static route' to point out the DSL interface. This is simply another default route, but with a higher administrative distance so that the leased line route is preferred unless the leased line is down. Search around for 'floating static route' and you should be able to find more information.
You can load share outbound traffic between the two links by not increasing the AD of the DSL route so that you end up with two equal default routes. You can then enable CEF if it isn't already and the router will automatically load balance outbound traffic on a per-flow basis. You'll probably need to NAT the source address of traffic that goes out the DSL interface so that the return traffic also comes through the DSL line. This isn't a concern with the leased line, as all inbound traffic to the IP block that you were assigned by the ISP will come in through that interface.
I'm sure someone here could provide specific commands if you post your current configuration, but I think your best bet would be to spend some time reading about this stuff so that you fully understand how it works when you implement it.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
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