My annual salary on my Earning and Leave Statement is much lower than what appears in box 1. Can I just manually insert my annual salary?

No. If your W2 is incorrect, you need to contact your employer to issue a corrected one. Your W2's are also filed with the IRS so they are expecting what you report matches what was reported to them.

No, you cannot alter your W-2.   If you think it is incorrect then have a discussion with the employer about box 1.  That W-2 is the same one that the employer sent to the IRS.  The amount you enter on your own tax return has to match the information the employer sent to the IRS.   And...you cannot attach statements to e-filed returns.