SAN FRANCISCO — Yahoo is giving its popular e-mail service a long-promised facelift in an attempt to make it more appealing to people who are increasingly using Facebook, Twitter, Google and other online alternatives.

The changes announced Tuesday build upon a redesigned e-mail format that Yahoo began testing seven months ago. The estimated 277 million users of Yahoo’s free e-mail service will be switched to the new version during the next few weeks.

The overhaul will enable updates to Facebook and Twitter accounts to be posted from within Yahoo’s e-mail boxes. The revamped service is supposed to be two times faster and capable of sending attachments of up to 100 megabytes.

Yahoo is counting on the changes to help attract and retain users at a time when more people are flocking to the rival Gmail service.

Although Yahoo’s service remains larger than Gmail, it has been losing ground in the past year. Through April, Yahoo boasted 277 million users worldwide, a drop of about 3 million, or 1 percent, from the same time last year, according to the research firm comScore Inc. Meanwhile, Gmail had grown to 220 million worldwide users, up 43 million, or 24 percent during the past year.

Most of Gmail’s gains appear to be coming at the expense of Microsoft’s Hotmail service, which remained the global leader with 327 million users, according to comScore. But that figure was down 27 million, or 8 percent, from last year.