Just to clarify, E+ seating is not an upgrade. Upgrade is from economy /economy plus cabin to first class cabin. As a silver you are quite unlikely to get a CPU (complimentary premier upgrade) so if you want first class seating either buy it or buy an economy fare and request the upgrade with points/miles and hope for the best. Be aware that UA usually delays processing mileage upgrades until shortly before the flight, so expect to stay in "waitlist' status for a while. If the requested upgrade does not clear you stay in the cabin and seat you selected at purchase.
If you must have an E+ aisle seat, then buy it at the time of your purchase. On popular, elite heavy routes E+aisle seats are often gone by the time you are checking in for your flight.